Recursively replace placeholders in CachedStrategy

Doesn't show an improvement in isolation, but will after ✨ Optimize placeholder retrieval from cache Active lands.

We can mutliple cache get the placeholders from all cached placeholders I think, and also implement recursion in case those placeholders also return placeholders.

Implemented the recursion. We don't have double-nested, cached, placeholders in core or at least not that I can think of, or that are covered by performance tests, but this does show the improvement for the single-nested cached placeholders in the navigation bar - e.g. shortcut and user menu placeholders now are now one multiple get instead of two gets.

Rebased 📌 Recursively replace placeholders in CachedStrategy Active which was postponed on this issue.

Tagging for release highlights because we should summarise the overall rendering performance gains between 11.1 and 11.2 somehow.

Back to green after ✨ Optimize redirect chain retrieval in VariationCache Active