Fix contrast in system-requirements callout

[janbrasna]

One-line summary

Uses the same callout as #15382, changes gray-30 to gray-20 for consistency.

Significant changes and points to review

Also removes:

• compact callout styling, that got removed with MZP v19 update and never adapted (so it's not used anymore?) see #14084/files
• logic to only show "Get the most recent…" on "supported systems" by comparing to body.is-modern-browser, as that logic also stopped working with MZP v19 update and has no function anymore.

Issue / Bugzilla link

Analogous to #15342

Testing

http://localhost:8000/firefox/123.0/system-requirements/