Cherry Pick PR's for WordPress 5.4 RC 3

[jorgefilipecosta]

Description

This PR cherry picks the following PR's into wp/trunk:

PR title	Author	Change required for cherry-picking
Components: Refactor SlotFill	@diegohaz	Update snapshots
Fix intermittent RichText e2e test failure	@youknowriad	Update snapshots
Fix: ResponsiveWrapper can not handle cases with small dimensions	@jorgefilipecosta	Use useResizeAware directly from react-resize-aware (package was already in use by the components package) instead of useResizeObserver from wordpress/compose (in trunk the hook is not available).
Fix: Cover block does not contain a class for predefined gradients	@jorgefilipecosta	-

cc: @youknowriad, @aduth, @gziolo

These PR's will be part of WordPress 5.4 RC 3.

How has this been tested?

I did some smoke tests and verified the branch, in fact, contained the expected fixes.

[aduth]

Looks good and tests well in some quick round of testing of the original issues 👍 Thanks for summarizing the conflict resolution notes as well.