Fix suggested_reviewers run when rails is disabled

What does this MR do?

Fix suggested_reviewers run when rails is disabled
    
The suggested reviewers feature/recipe depends on rails being
enabled. The recipe is now only ran when rails is enabled.
    
Fixes: https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/7479
    
Changelog: fixed

Related issues

Closes #7479 (closed)

Test Plan

version: '3.6'

services: 
  gitlab:
    image: 'gitlab/gitlab-ee:nightly'
    environment:
      GITLAB_OMNIBUS_CONFIG: |
        gitlab_rails['enable'] = false
    shm_size: '256m'
    volumes:
      - ./files/gitlab-cookbooks/gitlab-ee/recipes/default.rb:/opt/gitlab/embedded/cookbooks/gitlab-ee/recipes/

• setups crashes if modified gitlab-ee/recipes/default.rb is not mounted
• setup succeeds if modified gitlab-ee/recipes/default.rb is mounted

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened