qa/suites/crimson-rados/thrash: Modify osd_target_transaction_size

qa/suites/crimson-rados/thrash: Modify osd_target_transaction_size

We observed in Seastore, deletion of a large batch (default osd_target_transaction_size=30)
can take a significant amount of time.

Because this happens inside the peering_pp.process stage, it blocks the PG's peering pipeline.
During this block, any incoming OSDMap updates (PGAdvanceMap) are stalled behind the deletion work.
This eventually causes a global OSD-wide map progression hang because
the OSD cannot advance past an epoch until all PGs have processed
it.

To fix this, we are reducing osd_target_transaction_size to 5 to lower
conflict rates and allow deletion transactions to complete.

Fixes: https://tracker.ceph.com/issues/73791
Signed-off-by: Aishwarya Mathuria <amathuri@redhat.com>