Make use of uncached diffs where highlighting is not required

What does this MR do?

There are a few places that the diff highlighting cache is not required, and that's something we automatically fetch when calling Gitlab::Diff::FileCollection::MergeRequestDiff#diff_files. This MR focuses on replacing this by raw_diff_files call mainly for the new /diffs_metadata endpoint, where highlighting is definitely not required.

We can expect less Redis calls as an outcome.

closes #43654 (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