Remove roles count check from UserPermissions

[distantnative]

Description

• Merge Fix $user->roles($purpose) #6653 first
• Merge Clean up UserRules #6657 first

Summary of changes

• Remove roles count check from UserPermissions::canChangeRole()

Reasoning

It's not a permissions question but a UI thing to disable a button if only one role is available. Permission-wise, a user should be able to change a role even if only to the one available option.

This change is also needed to avoid a circular loop:

• $user->roles('change') filters the role collection by those that can be changed
• Check the permission to filter the roles collection
• The permission method itself would need to call $user->roles('change')

This is why this check should not be part of the permissions method.

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass