mds: update default MDS cache size memory limit

mds: update default MDS cache size memory limit

1GB is too low as a default and usually results in cache size warnings
at that size; the MDS will struggle to maintain such a small cache size
for most workloads.

Fixes: https://tracker.ceph.com/issues/43182
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>