Update diffs export for backwards compatibility

What does this MR do and why?

Mentions #227653 (closed)

In the recently merged MR !90159 (merged) we introduced a performance improvement for merge request externally stored diffs by caching those diffs locally during export. For this, we updated the import_export.yml file to export this information under a new key, diff_export.

I realised this approach is not backwards compatible, making exports from newer GitLab instances to older ones no longer work as before, since diffs information is now under diff_export key and not utf8_diff.

This MR:

• Updates utf8_diff method to utilize the new method that does caching, in order for the exported diff to remain under utf8_diff key in the exported json file
• From looking at the codebase, utf8_diff seems to be exclusively used during Project Export, making it easier to introduce caching in it, since there are no other users of this method. I didn't think about it in my previous MR. I was under the impression it's used elsewhere, but it does not look like it
• Resolves a follow up #367037 (closed) to log raised exception during diff_export method execution

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