Use CI_DEFAULT_BRANCH in Auto DevOps templates

What does this MR do?

We now have a CI_DEFAULT_BRANCH variable for use in our Auto DevOps CI templates, which allows us to code logic for any default branch, not just those named master.

This change was previously released with the intention that the variable would be interpolated under the ref key of only/except, but that didn't work so I reverted it. Now it exists under the variables key, where it seems to be working.

Screenshots

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

Availability 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

Closes #33601 (closed)