Remove Gin Admin Theme legacy fallbacks and modernizr fallbacks

Problem/Motivation

Since https://www.drupal.org/project/gin/issues/3309113 the gin admin theme 8.x-3.0-rc1 has not used the old CSS variables we use in our CSS as a fallback.

We should be able to remove legacy fallbacks in a minor release, as long as we note Gin Admin Theme compatibility.
We should also be able to remove obsolete .no-svg PNG icon fallbacks for toolbar buttons now that all modern browsers support SVG.

Steps to reproduce

Currently CSS for Gin Theme looks like this:

.gin--vertical-toolbar .toolbar-menu-administration > .toolbar-menu > .menu-item .toolbar-menu {
  margin-inline-start: calc(var(--gin-toolbar-width-collapsed, var(--ginToolbarWidthCollapsed)) - 4px);
}

.gin--vertical-toolbar[data-toolbar-menu="open"] .toolbar-menu-administration > .toolbar-menu > .menu-item .toolbar-menu {
  margin-inline-start: calc(var(--gin-toolbar-width, var(--ginToolbarWidth)) - 4px);
}

These two variables do not follow current drupal coding standards.
--ginToolbarWidth
--ginToolbarWidthCollapsed

And have been replaced upstream with

--gin-toolbar-width-collapsed
--gin-toolbar-width

Proposed resolution

If we need a fallback, we should use whatever they are using in the gin admin theme.

Remaining tasks

Create a merge request.

User interface changes

API changes

Data model changes