sched/feat: Use the new static key API for sched_feat

Both raw static keys and static_key_true/false() have been deprecated.
Migrate to new API.

It's a bit tricky to convert the sched_feat array to the new API because
the new one has two different types for true and false keys. Use a union
to wrap it.

No functional change.

Signed-off-by: Hongyan Xia <hongyan.xia@transsion.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260819081312.12447-1-hongyan.xia@transsion.com