mds/FSMap: assign v16.2.4 compat to pre-v16.2.5 standby daemons

mds/FSMap: assign v16.2.4 compat to pre-v16.2.5 standby daemons

With v16.2.5, the monitors store an MDS's CompatSet with its mds_info_t
in the MDSMap. If an older MDS fails and rejoins the cluster, it gets
assigned the empty CompatSet. This is problematic during upgrades as an
MDS failure may prevent the upgrade process from continuing and cause
file system unavailability.

This patch makes it so the mons will assign a reasonable default: a
CompatSet used since v14.2.0 until v16.2.5.

Fixes: https://tracker.ceph.com/issues/53150
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>