#65593: remove sitemap stylesheets

[masteradhoc]

Removes the sitemap XSL stylesheet feature from core. Chrome and other browsers are deprecating and removing XSLT support (https://developer.chrome.com/docs/web-platform/deprecating-xslt), so the <?xml-stylesheet?> processing instruction pointing at /wp-sitemap.xsl / /wp-sitemap-index.xsl will stop working for site visitors and no longer serves its purpose.

After this change, /wp-sitemap.xsl and /wp-sitemap-index.xsl 404, and generated sitemap XML no longer includes a stylesheet processing instruction.

Trac ticket: https://core.trac.wordpress.org/ticket/65593

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Implementing the removal across WP_Sitemaps, WP_Sitemaps_Renderer, canonical.php, and wp-settings.php, deleting the obsolete WP_Sitemaps_Stylesheet class, and updating/removing the associated PHPUnit tests. All changes were reviewed and tested by me.

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[masteradhoc]

@westonruter

This PR removes the XSLT-based sitemap stylesheet entirely. That aligns with Chrome's XSLT deprecation (https://developer.chrome.com/docs/web-platform/deprecating-xslt).

But: per Chrome's own docs, type="text/css" continues to be supported (see "XML + CSS is not being removed"). So there's an open question for core: do we want to keep some human-readable sitemap styling via a CSS-based , or is dropping styling altogether the intended outcome here?

[westonruter]

Good question. Is CSS able to style the XML sufficiently?

[masteradhoc]

@westonruter worth a try IMO. I'd though suggest we'll split it so we can first get rid of the existing implementation and then see if and what we want to style it moving forward.