docs(sql):sql: add POSTGRES_15

[CyberHippo]

Similar to #6069.
Align documentation with reality
Closes hashicorp/terraform-provider-google#14864

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

sql: added `POSTGRES_15` to available version docs

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[modular-magician]

Oops! It looks like you're using an unknown release-note type in your changelog entries:

• REPLACEME

Please only use the types listed in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md.

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I've detected that you're a community contributor. @hao-nan-li, a repository maintainer, has been assigned to assist you and help review your changes.

❓ First time contributing? Click here for more details

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Your assigned reviewer will help review your code by:

• Ensuring it's backwards compatible, covers common error cases, etc.
• Summarizing the change into a user-facing changelog note.
• Passes tests, either our "VCR" suite, a set of presubmit tests, or with manual test runs.

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[hao-nan-li]

/gcbrun

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 2 files changed, 4 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 2 files changed, 4 insertions(+), 4 deletions(-))
TF Conversion: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2780
Passed tests 2482
Skipped tests: 294
Affected tests: 4

Action taken

Found 4 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccDataSourceGoogleFirebaseAndroidAppConfig|TestAccComputeFirewallPolicyRule_multipleRules|TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample|TestAccDataSourceAlloydbLocations_basic

Get to know how VCR tests work

[modular-magician]

Tests passed during RECORDING mode:
TestAccDataSourceGoogleFirebaseAndroidAppConfig[Debug log]
TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample[Debug log]
TestAccDataSourceAlloydbLocations_basic[Debug log]

Tests failed during RECORDING mode:
TestAccComputeFirewallPolicyRule_multipleRules[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test

[hao-nan-li]

I don't think the detectors are reporting the right error messages here.

I'll confirm with the team and let you know.

[hao-nan-li]

/gcbrun

[hao-nan-li]

/gcbrun

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 2 files changed, 4 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 2 files changed, 4 insertions(+), 4 deletions(-))
TF Conversion: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2785
Passed tests 2487
Skipped tests: 295
Affected tests: 3

Action taken

Found 3 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccComputeFirewallPolicyRule_multipleRules|TestAccComputeNetworkEndpoints_networkEndpointsBasic|TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample

Get to know how VCR tests work

[modular-magician]

Tests passed during RECORDING mode:
TestAccComputeNetworkEndpoints_networkEndpointsBasic[Debug log]
TestAccApigeeKeystoresAliasesKeyCertFile_apigeeKeystoresAliasesKeyCertFileTestExample[Debug log]

Tests failed during RECORDING mode:
TestAccComputeFirewallPolicyRule_multipleRules[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test

[hao-nan-li]

LGTM