[ResponseOps][Cases] Removal of deprecated Cases APIs

[cnasikas]

In 8.1 we deprecate the following APIs:

• Route 1: GET /api/cases/{case_id}/comments
• Route 2: GET /api/cases/<case_id>/user_actions
• Route 3: GET /api/cases/status

For the following route, we deprecated the includeComments query parameter:

• Route 4: GET /api/cases/{case_id}

In 9.0, we should remove the deprecated APIs. Alternatives:

• For route 1 the alternative is GET /api/cases/{case_id}/comments/_find which was released in 7.13.
• For route 2 the alternative is GET /api/cases/<case_id>/user_actions/_find which was released in 8.7.
• For route 3 we will not offer an alternative.
• For route 4 the alternative is the same as for route 1.

The Cases UI uses the includeComments query parameter. We would need to update the Cases UI to use the alternative endpoint for the query parameter and extend the internal GET /internal/cases/metrics to return the stats needed by the UI.

The sub-issues describe the necessary steps needed to remove the deprecated APIs.

Related: #134344

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[jcger]

Should we add a subtask to delete the status endpoint from the code for v9?

[cnasikas]

I considered this issue to be the one for deleting the endpoints in v9. All sub-issued should be addressed before deleting the endpoints. Are there any concerns with this approach?