selinux: provide a "no sooner than" date for the checkreqprot removal

We marked /sys/fs/selinux/checkreqprot as deprecated in Linux v5.7,
but didn't provide any guidance as to the timeframe.  Considering
the state of checkreqprot, it seems like one year should be enough
time.

Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>