sched_ext: Replace bpf_ktime_get_ns() to scx_bpf_now()

In the BPF schedulers that use bpf_ktime_get_ns() -- scx_central and
scx_flatcg, replace bpf_ktime_get_ns() calls to scx_bpf_now().

Signed-off-by: Changwoo Min <changwoo@igalia.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>