Deprecate and remove freeze and unfreeze REST endpoints

[zuketo]

With the addition of a new Frozen Tier in Elasticsearch, and recent improvements in heap utilization, “frozen indices” (_freeze / _unfreeze endpoints) provide little end-user benefit.

Elasticsearch should deprecate, and then remove, _freeze and _unfreeze endpoints. This work should occur in a way that minimally impacts users, with a large amount of notice, and escape hatches along the way.

7.x:

• Deprecate freeze index endpoint (Deprecate freeze index API #72618)
• Deprecate the ILM freeze action (Adding deprecation logging and deprecation info API for ILM freeze ation #77969)
  - [ ] Warn on startup if frozen indices exist
• Add deprecation issue to deprecation info api that checks whether frozen indices exist in a cluster. (Add frozen index deprecation check. #78915)
• Warn when searching frozen indices? (Warn when searching a frozen index. #78184)
• Warn on use of ignore_throttled parameter? (Deprecate ignore_throttled parameter #77479)

8.0:

• Remove _freeze endpoint (no new frozen indices can be created) (Remove endpoint for freezing indices #78918)
• Make the ILM freeze action a no-op in 8.0 (Make the ILM freeze action a no-op #77158)
• Continue to support the _unfreeze endpoint throughout 8.x (give users an escape hatch for unfreezing existing frozen indices in their cluster)

9.0:

• Remove the _unfreeze endpoint in 9.0
• Automatically _unfreeze any frozen indices upgrading to 9.0
• Continue to retain the lazy engine internally for shared_cache searchable, for the Frozen Tier
• Add migration assistant support for inspecting for frozen indices and freeze actions in ILM policies

CC @jasontedor @ywelsch @dakrone

[elasticmachine]

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

[elasticmachine]

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

[jakelandis]

We met today to discuss how this removal intersects with REST API compatibility. In summary there is not really any way to apply compatibility here. The closest analog is searchable shapshots, but that has significantly different requirements and setup. The plan is to use REST API compatibility to help provide more meaningful errors. For example, rather then just a 404 returned, REST API compatibility can respond with some nice error message stating that freeze is no longer supported. This applies to both the freeze API's and the add/update ILM policies if they include a freeze action.