Fix GitLab Next badge styles

What does this MR do and why?

Adjusts the styles of the GitLab Next badge to be more closely aligned to gl-badge

Screenshots or screen recordings

Before :hover	After :hover

Before :focus	After :focus

How to set up and validate locally

• Turn on the Next badge by changing this line in app/views/layouts/header/_default.html.haml to say true

if Gitlab.com_and_canary?

if true

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.