fix: Improve progress bar color contrast in warning flash message

[avinashbot]

We had to revert this PR because Firefox didn't like it. Now fixed (see second commit) and tested in my dev pipeline, so we're good to go.

• Reverts cloudscape-design/components#2518
• Refactors the CSS to make it work with Firefox (split the -moz and -webkit rules into separate CSS declarations)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.84%. Comparing base (b922cf1) to head (a355d47). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2652 +/- ## ========================================== - Coverage 95.84% 95.84% -0.01% ========================================== Files 744 744 Lines 20581 20581 Branches 7015 7024 +9 ========================================== - Hits 19726 19725 -1 + Misses 847 800 -47 - Partials 8 56 +48 ```

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