Since mw-on-k8s now serves the majority of traffic, naming its X-Wikimedia-Debug option k8s-experimental doesn't make sense anymore.
I propose renaming it to k8s-mwdebug, and moving it to the top of the array to make it the default test server option.

On the functional side, this means changing debug.json in mediawiki-config and the x-wikimedia-debug-routing.lua configuration for trafficserver. For consistencty's sake, there are also a couple of replaces in Excimer's config and in wikitech documentation.