Security tech debt cleanup (#157990)

## Summary

This PR cleans up some outstanding tech debt.

### 1. Remove deprecation warning for `disabled` prop

This property was marked as deprecated the moment it was introduced (See
https://github.com/elastic/kibana/pull/118001/files#r753124740). However
it has been adopted by Fleet and Guided Onboarding plugins so I don't
think there's value in keeping this deprecation warning in place.

As an alternative we could also bump the `@removeBy` version if people
think we should still remove this property.


### ~~2. Remove deprecated property `requiredRoles`~~

~~This property has been deprecated and marked for removal for 8.8. The
property isn't being used anywhere in our codebase and we have reached
feature freeze for 8.8 so looks safe to be removed now.~~

Turns out this property is still used and can't be removed without
breaking existing functionality so reverting that commit.

x-pack/plugins/features/common/feature_kibana_privileges.ts

@@ -22,8 +22,6 @@ export interface FeatureKibanaPrivileges {
 
   /**
    * Whether or not this privilege should be hidden in the roles UI and disallowed on the API. Defaults to `false`.
-   * @deprecated
-   * @removeBy 8.8.0
    */
   disabled?: boolean;