Bump Spanner timeout from 4m -> 20m

[rileykarson]

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)

spanner: increased the default timeout on `google_spanner_instance` operations from 4 minutes to 20 minutes, significantly reducing the likelihood that resources will time out

[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, 6 insertions(+), 6 deletions(-))
Terraform Beta: Diff ( 14 files changed, 12 insertions(+), 16 deletions(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDatasourceGoogleServiceNetworkingPeeredDnsDomain_basic|TestAccCloudbuildWorkerPool_withNetwork|TestAccCloudFunctionsFunction_vpcConnector|TestAccComputeFirewallPolicyRule_update|TestAccComputeInstanceFromMachineImage_basic|TestAccComputeInstanceFromMachineImage_overrideMetadataDotStartupScript|TestAccComputeInstanceFromMachineImage_diffProject|TestAccComputeOrganizationSecurityPolicyAssociation_organizationSecurityPolicyAssociationBasicExample|TestAccComputeOrganizationSecurityPolicyRule_organizationSecurityPolicyRuleBasicExample|TestAccComputeOrganizationSecurityPolicy_organizationSecurityPolicyUpdateExample|TestAccComputeOrganizationSecurityPolicyRule_organizationSecurityPolicyRuleUpdateExample|TestAccComputeRegionNetworkEndpointGroup_regionNetworkEndpointGroupAppengineExample|TestAccContainerAnalysisOccurrence_multipleSignatures|TestAccContainerCluster_withWorkloadIdentityConfig|TestAccContainerCluster_withMasterAuthorizedNetworksDisabled|TestAccContainerNodePool_withInvalidUpgradeSettings|TestAccContainerNodePool_ephemeralStorageConfig|TestAccSqlDatabaseInstance_basicMSSQL|TestAccSqlUser_postgresIAM You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=213337

[modular-magician]

Tests failed during RECORDING mode: TestAccComputeOrganizationSecurityPolicy_organizationSecurityPolicyUpdateExample|TestAccComputeOrganizationSecurityPolicyRule_organizationSecurityPolicyRuleUpdateExample|TestAccContainerAnalysisOccurrence_multipleSignatures|TestAccComputeOrganizationSecurityPolicyAssociation_organizationSecurityPolicyAssociationBasicExample|TestAccComputeFirewallPolicyRule_update|TestAccContainerNodePool_ephemeralStorageConfig|TestAccCloudbuildWorkerPool_withNetwork|TestAccComputeInstanceFromMachineImage_overrideMetadataDotStartupScript|TestAccComputeInstanceFromMachineImage_basic|TestAccComputeRegionNetworkEndpointGroup_regionNetworkEndpointGroupAppengineExample|TestAccCloudFunctionsFunction_vpcConnector|TestAccComputeInstanceFromMachineImage_diffProject|TestAccSqlUser_postgresIAM Please fix these to complete your PR