Remove FF_ENABLE_PLACEHOLDERS

Follow-up from #619 (closed).

The flag should be removed in a few milestones, e.g. in %14.5

TODO

• Remove the feature flag from internal/feature/feature.go

• Remove its usage on internal/redirects/matching.go:48

• Remove relevant tests

• Update version history in docs - https://docs.gitlab.com/ee/user/project/pages/redirects.html#rewrites

  > - [Generally available](https://gitlab.com/gitlab-org/gitlab-pages/-/issues/620) in GitLab XX.X. Feature flag `FF_ENABLE_PLACEHOLDERS` removed.