Remove user-select: none from theme version string

[celloexpressions]

See ​https://make.wordpress.org/accessibility/2015/01/31/theme-switcher-accessibility-test-result/:

Why .theme-overlay .theme-version has user-select: none; ? As a user, I want to be able to select that text.

Let's remove those rules from wp-admin/css/themes.css.

[johneckman]

Patch removing these rules from themes.css and themes-rtl.css

[johneckman]

just themes.css this time - forgot about automatic generation of rtl version

[helen]

Came in with the merge, don't see any reason to keep it.

[helen]

In 31330:

Themes: Allow version number in the overlay to be selected.

props johneckman.
fixes #31205.