sched_ext: Add __COMPAT helpers for features added during v6.12 devel cycle

cgroup support and scx_bpf_dispatch[_vtime]_from_dsq() are newly added since
8bb30798 ("sched_ext: Fixes incorrect type in bpf_scx_init()") which is
the current earliest commit targeted by BPF schedulers. Add compat helpers
for them and apply them in the example schedulers.

These will be dropped after a few kernel releases. The exact backward
compatibility window hasn't been decided yet.

Signed-off-by: Tejun Heo <tj@kernel.org>