[7.x] Prevent searchable snapshots indices to be shrunk/split #75289
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Today if we try to shrink or to split a searchable snapshot
index using the Resize API a new index will be created
but can't be assigned, and even if it was assigned it won't
work as the number of shards can't be changed and must
always match the number of shards from the snapshot.
This commit adds some verification to prevent a snapshot
backed indices to be resized and if an attempt is made,
throw a better error message.
Note that cloning is supported since #56595 and in this
change we make sure that it is only used to convert the
searchable snapshot index back to a regular index.
Relates #74977 (comment)
Backport of #75227