Draft: Pull GitLab version value for stable releases

What does this MR do?

Update GitLab CI configuration to export GitLab version instead of revision for stable branches. This is required to resolve issues like #4944 (closed). GitLab QA images under registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa: are being cleaned up for older GitLab versions, so we need to switch to gitlab/gitlab-ee-qa:$QA_GITLAB_VERSION in such cases.

Related issues

Related to [CI] Pipeline on '7-0-stable' failed for commit... (#4944 - closed)

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
• When ready for review, follow the instructions in the "Reviewer Roulette" section of the Danger Bot MR comment, as per the Distribution experimental MR workflow

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 omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.