Revert "Do a sweep and remove most WARNS=6 settings"

Revert "Do a sweep and remove most WARNS=6 settings"

This reverts commit befc47822966, as it caused various CI build errors,
and we never merged the prerequisite commit fe815331bb40 ("build:
provide a default WARNS for all in-tree builds"). The latter is rather
intrusive, so I will avoid it.

Direct commit to stable/12 since the CI bots are broken.