Upgrade default theme to CoreUI v4 (which uses Bootstrap 5)

[tabacitu]

This is a demo of a Backpack v6 feature branch - provides bassets and a CoreUI v4 interface. Rephrased, it includes the following package PRs:

• https://github.com/Laravel-Backpack/CRUD/pull/4918
• https://github.com/Laravel-Backpack/PRO/pull/124
• https://github.com/DigitallyHappy/assets/pull/15

To get this running, in a demo project do:

git checkout bootstrap5
composer switched

Before we actually merge and launch this, we should:

• [ ] Delete all files in public/packages except the ones related to elFinder: rm -rf public/packages && php artisan backpack:filemanager:install (but not now... that would make the diff horrible)

[tabacitu]

The branch name here is very confusing, so let's close this in favor of https://github.com/Laravel-Backpack/demo/pull/472 - that's a copy of this branch, with a better name.