replace xml-sitemap with something newer

[bastianccm]

Last release was 6 years ago, and xmljs is prune to prototype pollution, we should update to a newer XML sitemap generator

[crunchy-vonage]

+1 This is blocking my repo commits, npm audit is reporting 3 critical security vulnerabilities

[mathiasschopmans]

The version 4.0.0 eliminated the generation of a sitemap.xml, but relies on superb static site generation via next.js.

Do sites really need a sitemap.xml?
Almost never.

https://www.imarc.com/blog/do-sites-really-need-a-sitemap-for-seo

If there's still a need for sitemaps, we may re-add this feature without any external dependencies. 🙏