Rename Gin

Problem/Motivation

Right before we merge the 6.x branch from here into core, the theme will be renamed. The name itself is yet to be decided. This will affect the following components:

• machine name
• namespaces in classes
• YML files in the project root: api, info, libraries. Maybe also the theme file if not gone by then
• all files in the config directory

Other filenames can be updated sooner. But the above list should only be done right before the final merge because otherwise, tests won't be possible any longer.