sched/debug: Rename sysctl_sched_min_granularity to sysctl_sched_base_slice

EEVDF uses this tunable as the base request/slice -- make sure the
name reflects this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230531124604.205287511@infradead.org