Refactor: Remove legacy hack in AcceptMergeRequest

My implementation of gitlab.WithRequestRetry made the code simpler 💪

c.f.

• https://gitlab.com/gitlab-org/api/client-go/-/releases/v0.129.0
• gitlab-org/api/client-go!2201 (merged)

TODO

• requires gitlab-org/api/client-go!2331 (merged)
• requires https://gitlab.com/gitlab-org/api/client-go v0.129.0+