Retain resource access token bot users after revocation

What does this MR do and why?

Part 1 of List Expired Project and Group Access Tokens (#462217)

This change is behind a new feature flag, retain_resource_access_token_user_after_revoke.

Resource Access Tokens (a.k.a. Group and Project Access Tokens) are implemented by creating "bot" users, giving it membership to the resource, and generating a Personal Access Token for that bot. When a Resource Access Token is revoked, the bot user is scheduled for deletion.

When the bot user is deleted the revoked token no longer has any relation to the resource. This makes it impossible to know what a revoked token had access to in the past.

With this change, when the FF is enabled, the bot user will be retained indefinitely.

In a subsequent MR we will add the UI to display information on revoked tokens.

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Related to #462217