[UII] Consider removing or refactoring agent count in confirmation modals

[jen-huang]

When we update settings objects, we display a count of affected policies and agents:

However this is not done all the time, for example updating proxy only shows a generic confirmation message without counts:

As relationships between policies and settings objects have gotten more complex (multiple agent policies, per integration outputs, space awareness), we should consider removing these granular counts in favor of a generic message to reduce the need for these deep lookups.

Alternatively, look into simplifying the logic and/or moving them server-side.

Original discussion: https://github.com/elastic/kibana/pull/189125#discussion_r1713870533