Shadow: remove support for shadow attribute in favour of using it in style

[madhusudhand]

Trac ticket: https://core.trac.wordpress.org/ticket/60377

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[youknowriad]

commit https://core.trac.wordpress.org/changeset/57510