[ILM] Warn user when shrink action has more shards in warm phase

[yuliacech]

This PR #84087 will add shrink action to the hot phase of an ILM policy.
Shrink action needs to have a number of shards configured. And it's possible to enable shrink in warm phase too.

If the warm phase is configured with a number of shards larger than the number of shards configured in hot phase, the ILM will fail because it can't shrink the index any further (see screenshot). We need to add a help text or warning to the input fields for shard numbers.

Also, we need to add a help text that number of shards must be a factor of the number of shards of the source index. For example, if the source index has 8 shards, it can only be shrunk to 4, 2 or 1 shard.
https://www.elastic.co/guide/en/elasticsearch/reference/master/ilm-shrink.html

Screenshot

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)