Make popup menu scrollbar customizable

[philippfromme]

Is your feature request related to a problem? Please describe.

The popup menu has a custom scrollbar that cannot easily be customized or reset to the defaults. We don't do this with any other scrollbars (e.g. properties panel).

Describe the solution you'd like

Make popup menu work with default scrollbar and allow customization.

Describe alternatives you've considered

Leave it as it is and let integrators figure out how to customize or reset to default.

Additional context

Related to https://github.com/camunda/product-hub/issues/2006

[nikku]

Moving to backlog as this is not a huge blocker right now.