Remove 'gitlab_default_can_create_group' setting

What does this MR do?

Remove rails 'default_can_create_group' setting

Remove the option gitlab-rails['default_can_create_group']
which was deprecated in 15.5.

Deprecation MR: https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/6316
Closes: https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/7217

Changelog: removed

Related issues

Closes #7217 (closed)

Test plan

• Verify gitlab_default_can_create_group is not rendered anymore

  docker run --rm -d --name gitlab registry.gitlab.com/gitlab-org/omnibus-gitlab/gitlab-ee:15.10.3-rfbranch.840675919.82589d47-0`
  docker exec gitlab cat /opt/gitlab/embedded/service/gitlab-rails/config/gitlab.yml | yq '.production.gitlab | with_entries(select((.key == "default_can_create_group")))'
  {}

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
  • gitlab-org/charts/gitlab#3821 (closed)
  • gitlab-org/charts/gitlab!3105 (merged)