Use full xhtml schema as required by Google

[psolbach]

While our sitemap validates without issues in most XML validators, the Google Search Console correctly reports an esoteric error with the namespacing (as rendered by Hugo).

Roughly: Our xmlns namespace requires all non-standard elements (like xhtml:link) to be declared in a schema instance namespace. Detailed explanation here. Even Google themselves do not declare namespace correctly in a tutorial.

This PR adds the required namespace information plus a noindex-nofollow fix from one of our projects.

[psolbach]

@peterschewe Removed the blocklist part. Otherwise, LGTY?

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