[BE] Graphql mutation to revoke fine-grained PAT

We need a mutation to revoke a personal access token.

Proposed mutation:

mutation {
  personalAccessTokenRevoke(id: "gid://gitlab/PersonalAccessToken/1") {
    errors
    personalAccessToken {
      ...
    }
  }
}

The personalAccessToken field is optional, though from an API design standpoint it would be nice to have so that info about the revoked token is available in the response.