Update MR form to use plural field names

The following discussion from !40330 (merged) should be addressed:

• @pedroms started a discussion: (+1 comment)

  @sming-gitlab 📝 Text: when the sidebar is empty for assignees, we already use the plural version “Assignees”:

  

  However, we don't do that in the MR form, we use the singular version:

  

  Would it be possible to update the field labels to Reviewers and Assignees?