fix: Restore Flashbar fixes with workaround for postcss-calc issue

[jperals]

Description

Due to some package consumers using an old version of postcss-calc which suffers from a parsing bug, #775 had to be reverted in #787 until a workaround was found. This PR reverts the revert and applies a workaround.

If we can arrive at the point where none of our consumers are affected by that issue, we can revert the second commit in this PR.

How has this been tested?

• I ran npm run quick-build and then ran postcss-calc 7.0.1 against the file lib/components/flashbar/styles.scoped.css. It crashed before the changes in this PR and succeeded with the changes applied.
• Visual regression tests for Flashbar fail as expected with the diffs being exactly the same as in fix: Stacked notifications in compact mode and without content padding #775
• Manually tested the page /app-layout/with-stacked-notifications-and-table. Comprehensive visual regression tests using this page can be introduced once this is merged

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

Base: 93.30% // Head: 93.30% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d0d21cd) compared to base (40f40f5).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #790   +/-   ##
=======================================
  Coverage   93.30%   93.30%           
=======================================
  Files         585      585           
  Lines       17135    17141    +6     
  Branches     4726     4732    +6     
=======================================
+ Hits        15988    15994    +6     
  Misses       1067     1067           
  Partials       80       80           

Impacted Files	Coverage Δ
src/app-layout/index.tsx	95.06% <ø> (ø)
src/app-layout/visual-refresh/notifications.tsx	100.00% <ø> (ø)
src/app-layout/notifications/index.tsx	100.00% <100.00%> (ø)
src/flashbar/collapsible-flashbar.tsx	97.95% <100.00%> (+0.05%)	⬆️

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