Enable syntax highlighting cache when rendering MR diffs

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Background

When we render MR diffs in rapid diffs, we aren't using the highlight cache when we render diffs on all cases (e.g. rendering first N diffs or streaming).

Proposal

Since we're re-using same highlighting logic, we should also use the caching feature it includes (Gitlab::Diff::HighlightCache).

In diffs_batch.json approach, we call Diff::FileCollection::MergeRequestDiffBase#write_cache. We should be able to use that as well.