Enable ETag caching for merge request notes polling

What does this MR do?

This was deferred because we needed a way to invalidate the cache when a note on a commit was added.

Now that these are in the DB, we can easily find related MRs and expire the etag.

This was discovered while investigating https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8445. Although this doesn't really fix that issue, I think it's a good change to make anyway to save some hits on the DB.

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

#17580 (closed)