Migrate group-level PAT expiry setting from Group Managed Accounts to Enterprise Users

We had group-level PAT expiry setting for group managed accounts. We deprecated and disabled group managed accounts, and migrated customers to use Enterprise Users.

However, this feature and (potentially a few others) still exist in the codebase specific to GMA. While it has been removed from the UI and documentation, the backend is still there. Having unavailable code in the codebase is not without drawbacks: this caused confusion / more complexity during Deprecate non-expiring access tokens (#369122 - closed).

Let's migrate this feature to Enterprise Users.