Kernel prepatch 6.12-rc1

Linus has released 6.12-rc1 and closed the
merge window for this release.

Despite conference travel (both for me and several maintainers),
things seemed to go mostly fairly normally. There's a couple of
notable new features in here: For one thing, PREEMPT_RT is now
mainlined and enabled as a config option (you do need to enable
'EXPERT' to get the question). For another, sched_ext also got
merged.