Support deleting Firestore databases

[modular-magician]

Due to backwards compatibility concerns, the default behavior remains to abandon the database upon destroy rather than to actually delete it.

To actually delete the database, you must set deletion_policy to DELETE, and apply if necessary, before running terraform destroy.

This also cleans up some related deletion-related docs and bugs:

• Updates the delete protection docs
• Either delete protection or deletion policy being set prevents destroy

Fixes #16488

Fixes #16404

Fixes #16325

Release Note Template for Downstream PRs (will be copied)

firestore: enabled database deletion upon destroy for `google_firestore_database`

firestore: added virtual field `deletion_policy` to `google_firestore_database`

firestore: prevent destruction if both `deletion_policy` is `DELETE` and `delete_protection_state` is `DELETE_PROTECTION_ENABLED`

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Manual Testing notes:

delete_protection_state == enabled, policy == abandon, result: abandon
delete_protection_state == enabled, policy == delete, result == prevent
delete_protection_state == disabled, policy == abandon, result == abandon
delete_protection_state == disabled, policy == delete, result == delete

Derived from GoogleCloudPlatform/magic-modules#9450

[github-actions]

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