Fix encoding error in MR diffs

What does this MR do?

Fix encoding error in MR diffs when using external diff storage.

You need to clear diff highlight cache if you are using external diff storage. You can clear diff highlight cache as described below.

1. Enter rails console

sudo gitlab-ctl console

2. Clear cache

MergeRequestDiff.in_batches do |merge_request_diffs|
  merge_request_diffs.where(stored_externally: true).each do |merge_request_diff|
    cache = Gitlab::Diff::HighlightCache.new(merge_request_diff.diffs)
    cache.clear
  end
end

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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

Performance 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 #61841 (moved)