Admin: Remove alt syntax from dashicons.css

Safari only introduced support for alternative text in generated CSS content in version 17.4, released in 2024. This meets the latest two version declared for core browser support. But because `dashicons.css` is commonly required on the front-end for plugins and themes, this support impacts front-end users, and the browser support guidelines for core should not apply in this context.

Follow up to [60885].

Reviewed by desrosj.
Merges [61290] to the 6.9 branch.

Props codex-m, sabernhardt, wildworks, joedolson, desrosj, welcher.
Fixes #64266.

git-svn-id: https://develop.svn.wordpress.org/branches/6.9@61293 602fd350-edb4-49c9-b593-d223f7449a82

Author: joedolson