Incorporate module specific changes that get removed from Gin

Problem/Motivation

The Gin admin theme will be merged into Drupal Core 11.3, see #3530852: [META] Admin theme modernisation

In preparing for that change, Gin has to remove contrib specific code, and therefore this module should incorporate those removed changes itself. Past and current versions of the Gin admin theme are not affected by this, they continue to contain that contrib specific code.

Proposed resolution

Please collect the changes from this MR and bring that code back into this module.