WIP: [EE] Use the only.changes syntax in .gitlab-ci.yml

EE port of: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27282

What does this MR do?

This simplifies the logic greatly, and remove the need for the specific branch names.

The only thing is that I cannot test for a documentation-only change as I changed the .gitlab-ci.yml file which triggers all the code-changes-related jobs... :)

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team