osd/PG: do not use approx_missing_objects pre-nautilus

osd/PG: do not use approx_missing_objects pre-nautilus

We changed async recovery cost calculation in nautilus to also take
into account approx_missing_objects in ab241bf7e927cda2d0ed1698383d18dc4a4b601c
This commit depends on https://github.com/ceph/ceph/pull/23663, hence
wasn't backported to mimic.

Mimic only uses the difference in length of logs as the cost. Due to this,
the same OSD might have different costs in a mixed mimic and nautilus(or above)
cluster. This can lead to choose_acting() cycling between OSDs, when trying
to select the acting set and async_recovery_targets.

Fixes: https://tracker.ceph.com/issues/39441
Signed-off-by: Neha Ojha <nojha@redhat.com>