Add validation of the number_of_shards to the beginning of the ILM shrink step

[joegallo]

The ILM shrink action can't succeed if the requested number_of_shards won't work with the index's number_of_shards. An example of that would be if the index currently has 3 shards, the policy tries to shrink it down to 2 shards.

Rather than running the set-single-node-allocation allocation step successfully and then failing at the shrink step, we should have validation that checks for this scenario and fails the shrink action without having moved the shards around.

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[joegallo]

Related to #56062 because this could be one of the potential failure cases mentioned there.

[andreidan]

Fixed by #76565