Remove messages from always loaded libraries in Claro and starterkit

Problem/Motivation

Discovered via #3565258: Support library-specific aggregates.

Both starterkit and Claro specify the messages library to be always loaded via .info.yml, Olivero does not.

The messages library is added to the page when there are messages to render, either directly in PHP or via the AJAX messages command.

There is one additional case where messages can be set - directly via the js messages API, but in those cases the library that's doing that could add the messages library as a dependency.

This should reduce unused CSS. We can also set css_aggregate_target: true for the messages library once this is done, since it will often be the only different library that's loaded on a page.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet