sql: support in-place storage shrink for google_sql_database_instance

[DrFaust92]

Summary

Reducing disk_size on google_sql_database_instance (with disk_autoresize disabled) previously forced the instance to be recreated — a destructive operation for a data disk. Cloud SQL now supports reducing storage in place via the dedicated Instances.PerformDiskShrink operation (GA for MySQL/PostgreSQL, and available for SQL Server).

This change wires that up:

• diskSizeCutomizeDiff no longer calls ForceNew on a shrink; the shrink is kept as an in-place update.
• On update, a disk_size decrease is carried out with Projects.Instances.PerformDiskShrink (the settings update keeps the current size, so it's a no-op for disk).
• A pre-flight GetDiskShrinkConfig call returns a clear error when the requested size is below the instance's current minimum shrinkable size, instead of letting the shrink operation start and then fail.
• disk_size field docs updated.

Increases are unaffected (still applied via the settings update). A shrink with disk_autoresize enabled continues to be ignored, as before.

Tests

Acceptance tests were run against real GCP:

• TestAccSqlDatabaseInstance_storageShrink (new) — creates a 100GB instance and shrinks it in place to 90GB, asserting the disk size and that the instance is not recreated (same ID). PASS
• TestAccSqlDatabaseInstance_DiskSizeAutoResizeWithDiskSize (updated) — the step that previously asserted a forced recreate now asserts an in-place update plan (PlanOnly + ExpectNonEmptyPlan). PASS

Note: storage shrink is not supported on shared-core machine types, so the new test uses a db-custom-* tier.

Release Note

sql: `google_sql_database_instance` now performs an in-place storage shrink when `disk_size` is reduced with `disk_autoresize` disabled, instead of forcing instance recreation

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

@shuyama1, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

You can help make sure that review is quick by doing a self-review and by running impacted tests locally.