Use $CI_DEFAULT_BRANCH in Pages CI templates

What does this MR do?

• Adds tests for only:/except: keys for jobs defined in the Pages CI templates
• Replaces uses of master with $CI_DEFAULT_BRANCH == $CI_COMMIT_REF_NAME

Since previous attempts (!19454 (merged), !18915 (merged)) to move from master to CI_DEFAULT_BRANCH have broken Auto DevOps, I'm going to perform the transition in multiple stages. This MR, which only migrates the Pages templates, is the first stage.

Hopefully having unit tests for the templates mitigates the risk posed by the transition.

Issue: #33601 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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