Reduce specificity of global styles and layout selectors.

[tellthemachines]

Trac ticket: https://core.trac.wordpress.org/ticket/61165

Syncs changes from WordPress/gutenberg#60106, WordPress/gutenberg#60228 and WordPress/gutenberg#61340.

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[tellthemachines]

Just realised that changes from #6489 are likely to conflict with these, especially the test string updates, so I might hold off on updating this one until #6489 is committed.

[github-actions]

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[tellthemachines]

PHP tests are updated, so marking this ready for review!

[github-actions]

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[aaronrobertshaw]

There are some proposed changes to specificity in WordPress/gutenberg#61638 and a general consensus to proceed with those changes for WP6.6.

Given the tight timeline until the 6.6 beta I've put together a separate draft PR bringing together all the specificity changes from the following PRs:

• Try reducing specificity of global styles selectors only. gutenberg#60106
• Try reducing specificity of layout style selectors gutenberg#60228
• Reduce specificity of global styles body margin reset rule. gutenberg#61340
• Styles: try wrapping with :root to fix reset styles gutenberg#61638

Once they settle that could be rolled into this PR or the core ticket updated to point to the alternate backport.

[tellthemachines]

Closing in favour of #6633.