Revert "Remove $wgCategoryTreeDynamicTag = true"

This reverts commit b77a74f0eab759fae90ae58ddb3f97481a05385b.

Just a long shot, seeing if it's the cause of raised poolcounter congestion and/or s6 page restrictions query durations, which commenced around the same time.

Change-Id: Ib336530c8bf1c6f9ef239b5890f69c200d0cc48e
Merge "Revert "Remove $wgCategoryTreeDynamicTag = true""