[ILM] Forcemerge action + new default merge policy

[jakelandis]

Based on the outcome of #31742 [1] we may need to make some modification to the Forcemerge action. For example may want to add a "force" option to allow overriding the index defaults, or allow an option to change the index defaults.

There is isn't much to do now, and this issue is a reminder to revisit once #31742 has been resolved.

[1]
Lucene just added a change to the TieredMergePolicy (default merge policy) which prevents it from force-merging indices down to 1 segment if this wouldn't meet the requirement that all segments must be less than the maximum segment size, 5GB by default. )

[elasticmachine]

Pinging @elastic/es-core-features

[jakelandis]

Closing as nothing to do here. #31742 was resolved with #32291 which keeps behavior as-is.