Add state migration for federated_identity_credential parent_id deprecation

[utafrali]

Fixes #32074

Description

The deprecation of parent_id in favor of user_assigned_identity_id was breaking for users with existing state. Added a state migration to handle the upgrade path so existing resources don't break when users upgrade the provider.

PR Checklist

• Followed Contributing Documentation
• Updated/added Documentation
• Written new tests

Changes to existing Resource / Data Source

Updated FederatedIdentityCredentialResource to implement ResourceWithStateMigration and added the StateUpgraders() method that registers the v0 to v1 migration. The migration logic copies the value from parent_id to user_assigned_identity_id.

Testing

Added TestAccFederatedIdentityCredential_parentIdToUserAssignedIdentityId which verifies the migration works by creating a resource with the old config and upgrading to the new one. The test is gated with the feature flag so it only runs on pre-5.0 versions.

Change Log

N/A

Related Issue(s)

Closes #32074

AI Assistance Disclosure

N/A

[sreallymatt]

Hi @utafrali - Thanks for your interest in contributing to the provider!

While this change does technically resolve the linked issue, since this is only a problem when running a plan/apply operation without a refresh step (i.e. terraform plan -refresh=false), we are not going to merge this state migration.

State migrations are one-way only, meaning as soon as this is merged, users will no longer be able to downgrade to a previous minor version of the provider which is arguably a bigger issue than needing to run a plan/apply operation with a refresh (which the majority of users likely already do)

Because of this, I'm going to close this PR.