osd_types.cc: reorder encoding/decoding of fields in pg_stat_t

osd_types.cc: reorder encoding/decoding of fields in pg_stat_t

Fields state, purged_snaps and snaptrimq_len are new to Mimic.
Reorder them in a way that newest field (snaptrimq_len) is before
two others and uses other encoding version, so the pull request
https://github.com/ceph/ceph/pull/19520 can be backported to Luminous
without breaking Luminous -> Mimic upgrade.
This also changes encoding/decoding version back to 24 as both state
and purged snaps were added post-Luminous and pre-Mimic, so we can
push it into a single struct version and keep snaptrimq_len into other
version.

Signed-off-by: Piotr Dałek <piotr.dalek@corp.ovh.com>