Convert muted badges to neutral

What does this MR do and why?

Converts muted badge variants to neutral (details in Deprecate neutral muted badge color (gitlab-org/gitlab-services/design.gitlab.com#2024 - closed))

Related: Remove muted badge variant (gitlab-org/gitlab-services/design.gitlab.com!4977 - merged)

Screenshots or screen recordings

Examples from comments and the commits list:

Before	After

Links

• ✂️ UX Paper Cuts 18.4 → AI (#548836 - closed)
• Deprecate neutral muted badge color (gitlab-org/gitlab-services/design.gitlab.com#2024 - closed)