Accessibility: Improve readability by removing unnecessary italic font style.

Continues the removal of italic text where unnecessary or where used on whole lines and big chunks of text.
Removes italic type from:
- all the `howto` text e.g. the "Tags" meta box, the "Featured image" meta box, the "Insert/edit link" modal dialog in the Classic Editor 
- the Taxonomies list pages
- the edit Taxonomies pages
- the Menus page
- the Settings pages

Props audrasjb.
See #47327.

Built from https://develop.svn.wordpress.org/trunk@48116


git-svn-id: https://core.svn.wordpress.org/trunk@47885 1a063a9b-81f0-0310-95a4-ce76da25c4cd