ItemGroup: Focus outline in borderless usage does not match standard outline

[mirka]

What problem does this address?

When ItemGroup is used with the border-free settings, the focus outline on Items are noticeably thinner compared to other visually similar components.

ItemGroup

NavigatorButton

Button

What is your proposed solution?

Make the ItemGroup focus outlines match the standard outline styles, at least for the border-free variant.

[ciampo]

Added to #34709

I think we soon have to have a deeper look at ItemGroup and iron out as many issues as possible, it seems like it will become an important component for our UIs