Documenting wording for passing as environment variable GITLAB_VERSION

What does this MR do?

Adding the wording in the documentation for passing gitlab_version in the vars.yml as an Environment variable GITLAB_VERSION .

Reference MR - !1219 (merged)

## Related issues

#817 (closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
  • Code:
    • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
    • Documentation created/updated in the same MR.
    • If this MR adds an optional configuration - check that all permutations continue to work.
    • For Terraform changes: set up a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.