Fix ETag caching not being used for AJAX requests

What does this MR do?

This fixes two issues:

1. For AJAX requests, we shouldn't set the no-store response header as this prevents the browser from sending the If-None-Match request header (with the Etag value), leading to poor performance. This was broken since 11.5.0 due to a security fix (see issue).
2. Ci::Pipeline#all_merge_requests was only returning merge requests for its project, but it should include merge requests from forks as well. This was preventing Etag value from being touched in ExpirePipelineCacheWorker.

What are the relevant issue numbers?

Closes #57905 (closed).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer