Prevent almost all significant event rule mutations

[kobelb]

Significant event rules are created automatically when the user defines a significant event for a stream. #221382 will block the creation of these significant event alerting rules.

However, we also need to prevent almost all mutations of these significant event rules. The only action that we want enabled the ability to update the API key. No other mutations, including edit, should be available to users via the UI or the HTTP APIs.

Definition of Done

• UI is updated to only allow the user to update the API key for significant event rules. No other mutations (edit, mute, etc.) should be available.
• HTTP APIs are updated to only allow the user to update the API key for significant event rules. No other mutations (edit, mute, etc.) should be possible. A 400 Bad Request http status code should be returned.

[elasticmachine]

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

[cnasikas]

PR for the update APIs: #234937