Add CI variable to provide GitLab FQDN

What does this MR do?

Add a predefined CI variable that stores the FQDN of GitLab instance (like gitlab.com or dev.gitlab.org)

Development timeline

1. Decided to use HOST instead of FQDN. IP addresses are valid values for GitLab's host address, and calling them FQDN is not technically correct.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes: #64161 (closed)