Use wikimedia/less.php instead of leafo/lessphp #16813

Description:

The resulting CSS seems to change slightly, but seems everything still works as expected.
Using this library also fixed an importing issue for custom plugin dirs that should have been visible through the now changed UI file 🙈

fixes #16082

Review

• [ ] Functional review done
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done see checklist
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed