Add PSQL 13 to docs and change defaults on SQL Version field

[upodroid]

Fixes hashicorp/terraform-provider-google#7772
Fixes hashicorp/terraform-provider-google#5212

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).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• 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).
• Ran relevant acceptance tests (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: the default value for `database_version` of `google_sql_database_instance`has been changed to MYSQL_5_7 . Please update your configuration to match this.

[modular-magician]

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

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@c2thorn, please review this PR or find an appropriate assignee.

[upodroid]

I'm aware that this change might cause issues for the 0.0001 % of terraform users who forgot to specify mysql5.6 as their version and now need to set the field.

There is a fat warning here about the API being in beta and receiving backward incompatible changes.
https://cloud.google.com/sql/docs/mysql/admin-api

I changed the default because google is currently defaulting to 5.7 which you can verify by running

echo '{"kind": "sql#instance", "name": "foo-11111", "project": "REDACTED", "region": "us-central", "settings": {"kind": "sql#settings", "pricingPlan": "PER_USE", "replicationType": "SYNCHRONOUS", "tier": "db-n1-standard-1"}}'| http POST https://sqladmin.googleapis.com/sql/v1beta4/projects/REDACTED/instances "Authorization: Bearer $(gcloud auth print-access-token)"

gcloud sql instances list --format yaml

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

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

[c2thorn]

Hey @upodroid this should be saved for the next major release. I've made an issue here to track, and we can revisit this when it comes time.

[upodroid]

Fixes points to the correct issue now

[c2thorn]

@upodroid We will likely just make this field required instead of updating the default every time. Currently @shuyama1 is assigned, so she'll likely take that up.

If you think that's acceptable, I think we can close this.

[upodroid]

Yeah, make it required. It makes the resource less fragile.

[c2thorn]

Thanks @upodroid.