Sync from kernel to receive new vmlinux.h and the updates to common headers.
This includes the following updates:
- scx_bpf_switch_all() is replaced by SCX_OPS_SWITCH_PARTIAL flag.
- sched_ext_ops.exit_dump_len added to allow customizing dump buffer size.
- scx_bpf_exit() added.
- Common headers updated to provide backward compatibility in a way which
hides most complexities from scheduler implementations.
scx_simple, qmap, central and flatcg are updated accordingly. Other
schedulers are broken for the moment.
- combine c and kernel-examples as it's confusing to have both
- rename 'rust-user' and 'c-user' to just 'rust' and 'c', which is simpler
- update and fix sync-to-kernel.sh
