Change the default for google_sql_database.deletion_policy to DELETE

[modular-magician]

Fixes hashicorp/terraform-provider-google#13206

As mentioned in hashicorp/terraform-provider-google#13206 (comment), this change is technically a breaking change- but it's also fixing a regression in the resource, so it's being made in a minor version anyways. It will result in a diff, which I've called out in the changelog entry (intentionally included twice, as a bugfix and a note)

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # google_sql_database.database will be updated in-place
  ~ resource "google_sql_database" "database" {
      ~ deletion_policy = "ABANDON" -> "DELETE"
        id              = "projects/graphite-test-rileykarson/instances/myinstance/databases/mydb"
        name            = "mydb"
        # (5 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

google_sql_database.database: Modifying... [id=projects/myproject/instances/myinstance/databases/mydb]
google_sql_database.database: Modifications complete after 1s [id=projects/myproject/instances/myinstance/databases/mydb]

This wasn't caught by our tests because the testcheckdestroy runs after terraform destroy, and deleting the instance deleted the database.

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: fixed an issue where `google_sql_database` was abandoned by default as of version `4.45.0`. Users who have upgraded to `4.45.0` or `4.46.0` will see a diff when running their next `terraform apply` after upgrading this version, indicating the `deletion_policy` field's value has changed from `"ABANDON"` to `"DELETE"`. This will create a no-op call against the API, but can otherwise be safely applied.

sql: fixed an issue where `google_sql_database` was abandoned by default as of version `4.45.0`. Users who have upgraded to `4.45.0` or `4.46.0` will see a diff when running their next `terraform apply` after upgrading this version, indicating the `deletion_policy` field's value has changed from `"ABANDON"` to `"DELETE"`. This will create a no-op call against the API, but can otherwise be safely applied.

Derived from GoogleCloudPlatform/magic-modules#6936