Replace gitlab-rdoc with rdoc

What does this MR do?

We had to fork rdoc in order to upgrade to 6.3.1 because the release was missing a file. Now that a fixed version has been released, we can switch back to it. See #334695 (closed).

The diff is very minimal since it is just correcting the issue with the previous release: https://my.diffend.io/gems/rdoc/6.3.1/6.3.2/

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.