Styles: Use HTML API to generate <style> tags

[sirreal]

The HTML API correctly escapes the content and attributes of STYLE tags and is preferable to other methods of creating the tags like simple string concatenation.

This allows for richer CSS to be embedded in STYLE tags without sacrificing safety, like the example CSS in ticket 64418:

@property --animate {
  syntax: "<custom-ident>";
  inherits: true;
  initial-value: false;
}

This does not fix the ticket, but is an important part of the solution to ensure that arbitrary CSS can be safely embedded inside of HTML STYLE tags.

Trac ticket: https://core.trac.wordpress.org/ticket/64418

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[github-actions]

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