…le snapshot action (elastic#137375)

In elastic#133954, we modified ILM's searchable snapshot action to perform the
force-merge on a clone of the index with 0 replicas. This optimization
avoids performing the force-merge redundantly on replicas, as the
subsequent snapshot operation only looks at primary shards.

We've seen some cases where cloning the index resulted in issues; there
was a bug in the clone API that caused shards to be initializing
permanently under specific circumstances (fixed by elastic#137096), and cloned
shards are unable to be assigned if their source lives on a node that is
close/past the low watermark disk threshold (will be fixed soon by the
Distributed Coordination team).

Therefore, we implement an opt-out flag that users can configure in the
`searchable_snapshot` action of their ILM policy if they don't want to
clone the index with 0 replicas before performing the force-merge. We
implement an opt-out instead of an opt-in, as we believe these issues to
be rather specific (and soon resolved), and the clone is worth doing by
default.

(cherry picked from commit 0ab3240)

# Conflicts:
#	server/src/main/resources/transport/upper_bounds/9.2.csv
#	server/src/main/resources/transport/upper_bounds/9.3.csv
#	x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/ilm/action/RestPutLifecycleAction.java