[SRU] Backport Gnome Shell high-contrast dock fix

[Paul]

[Impact]

 * When switching to High Contrast on Ubuntu Impish, the Ubuntu Dock detaches from the margin, changes look and expands.

 * Backport the patches to supported versions of Ubuntu will fix the user experience, and will polish the desktop look and feel.

[Test Plan]

 * Open "Settings" app;

 * Go to "Accessibility" page;

 * Enable "High Contrast" option with the switch;

 * See dock problem;

 * Related launchpad bug: https://bugs.launchpad.net/ubuntu/+source/yaru-theme/+bug/1946042

[Other Info]

 * This issue affects all supported versions of Ubuntu:

   - The original patch has been pushed here: https://github.com/ubuntu/yaru/pull/3089
   - A patch for "focal" has been pushed here: https://github.com/ubuntu/yaru/pull/3223
   - A patch for "impish" has been pushed here: https://github.com/ubuntu/yaru/pull/3224

 * The problem was that the High Contrast variant of Yaru wasn't import dock styling.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.