video_core: Make use of ordered container contains() where applicable

With C++20, we can use the more concise contains() member function
instead of comparing the result of the find() call with the end
iterator.

src/video_core/buffer_cache/buffer_cache.h

@@ -545,7 +545,7 @@ private:
     bool IsRegionWritten(VAddr start, VAddr end) const {
         const u64 page_end = end >> WRITE_PAGE_BIT;
         for (u64 page_start = start >> WRITE_PAGE_BIT; page_start <= page_end; ++page_start) {
-            if (written_pages.count(page_start) > 0) {
+            if (written_pages.contains(page_start)) {
                 return true;
             }
         }