Use library version as query strings for external css (like js does)

Non aggregated CSS assets use a value from state which is derived from a timestamp and updated on cache clear. There is no need to append a version parameter as far as I can see? Honestly I'm not sure if JS files need it either? Marking PMNMI in case anyone disagrees

I think this is valid, but it's a task rather than a bug.

Using the version instead of the hashed timestamp means that if the CSS version doesn't change, it can persist in CDN/varnish/browser caches even when the Drupal cache is cleared which should result in better hit rates.

If we have tests for the js version in the query string, we should be able to adapt those for CSS straightforwardly so adding the 'needs tests' tag.