text: Title: Remove `kibana.index`, `xpack.reporting.index`, `xpack.task_manager.index` settings
  Body: As discussed in https://github.com/elastic/kibana/issues/82020, we will be removing the following settings in 8.0:
  - [x] `kibana.index` - https://github.com/elastic/kibana/pull/112773
  - [x] `xpack.reporting.index` - https://github.com/elastic/kibana/pull/113803
  - [x] `xpack.task_manager.index` - https://github.com/elastic/kibana/issues/102076
  
  These settings have already been deprecated in 7.x; however, they have not been removed from `master`.
  
  The Observability team currently takes advantage of these settings to allow their developers to use a multi-tenant Cloud environment. They are actively testing a few approaches to do so, but haven't implemented one yet. As a result, we should delay removing these settings until they have a path forward.
  
  Blocked by https://github.com/elastic/observability-test-environments/issues/915