feat(NotificationBadge): add support for plain variant

[kmcfaul]

What: Closes #12066

Currently I've added support for "plain" based on the issue's usage (adding it to the existing variant prop), but I'm wondering whether this may be better as a separate isPlain flag. Though the NotificationBadge props are not 1:1 with Button's props, so maybe adding it as part of the variant prop is still fine. LMK if anyone has thoughts or preferences about one method or another.

[patternfly-build]

Preview: https://pf-react-pr-12139.surge.sh

A11y report: https://pf-react-pr-12139-a11y.surge.sh

[thatblindgeye]

An isPlain flag might slightly make more sense? As a value "plain" is pretty different from the other variant values, though at the same time we'd most likely have the logic setup the way it is in the PR now (I don't think we'd want a plain NotificationBadge to have the "attention" icon for example). I think it's good as-is, though.

[mcoker]

Did we ever get design approval? I see the question in #12069 but no response

[mcoker]

LGTM the goal is just a fourth variant where it's always plain.

IMO an isPlain prop might imply you can still use read/unread/attention variants? We can't do it now, but could with a small change.