service: do fine-grained validation of CPUSchedulingPriority= at execution time

service: do fine-grained validation of CPUSchedulingPriority= at execution time

The precise bounds of the scheduling priority depend on the scheduling policy,
so depending on the order in which the two settings are specified the
validation might pass or fail.
When checking the setting only validate the outer range (valid values in general are 0 to 99),
and let the execution fail later if the priority does not match the
specified policy (1 to 99 for RR/FIFO, 0 for the rest).

Fixes https://github.com/systemd/systemd/issues/20320