[Index Management] Remove the "unfreeze index" action

[yuliacech]

The freeze index API was removed in Elasticsearch v8.0 (docs). We removed the freeze index action in Index Management in #116110 but we left an option to unfreeze any frozen indices left over from versions 7.x. At some point we could delete the unfreeze action from the UI too. It would still be possible to unfreeze frozen indices via the Elasticsearch API (docs).

The unfreeze index action is in the context menu (index details page/flyout, indices list) and there is Kibana API /index_management/indices/unfreeze.

[elasticmachine]

Pinging @elastic/platform-deployment-management (Team:Deployment Management)

[elasticmachine]

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

[alisonelizabeth]

Completed via #205617