Replace gitlab-rdoc with rdoc

As part of the security fix for https://gitlab.com/gitlab-org/gitlab/-/issues/329904, we had to create a fork of rdoc in order to get a working version that had the vulnerabilities mitigated.

Once rdoc releases a working version newer than 6.3.1 (which is missing a file), we should go back to using that instead of the gitlab-rdoc fork.

See github.com/ruby/rdoc/issues/808 and github.com/ruby/rdoc/pull/809 for details about the problem with rdoc 6.3.1.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/329904 and https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/1531