Add Gitlab version and revision to export

Creates

1. GITLAB_VERSION: that is a context of Gitlab::VERSION.
2. GITLAB_REVISION: that is a context of Gitlab.revision.

This will give us more flexibility of supporting future exports of GitLab, as it could allow us to switch to GITLAB_VERSION instead of using VERSION as we are doing today.

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

#118640 (closed)