Deprecate the social links Core pattern

[mikachan]

This PR adds a deprecation notice to the "Social links with a shared background color" pattern from Core, which addresses the discussion in this Gutenberg issue: WordPress/gutenberg#55572.

Trac ticket: https://core.trac.wordpress.org/ticket/61708#ticket

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Props mikachan, peterwilsoncc.

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[peterwilsoncc]

This looks good to me and tests well.

I created a post using the pattern on trunk and can confirm it continues to work both in the editor and on the front end with this branch checked out.

I've taken the liberty of merging trunk in to this PR to include the recent commits.

[peterwilsoncc]

Merged r59019 / a78540b

[mikachan]

Thanks so much, @peterwilsoncc!