chore(plugin-sitemap): Update sitemap package 📝

[sanjaiyan-dev]

I've put together a small PR to update the sitemap dependency from ^7.1.1 to ^9.0.1. I noticed the older version is missing some recent security patches, so I'm hoping this update is helpful!

Key improvements in this bump:

• Security Patches: Resolves vulnerabilities including XML injection, memory DoS, and arbitrary file writes (v8.0.1 & v9.0.1).
• Node.js Fixes: Resolves a circular dependency issue that breaks Node 20.6+ (v7.1.2).

⚠️ Important Note:
Version 9.0.0 drops support for Node.js < 20 (it requires >=20.19.5).

Extremely sorry if I made any mistakes :)

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

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[slorber]

It's not possible yet to upgrade to v9 since it has breaking changes for Docusaurus v3 users (requiring a nodejs version newer than 20.0 that we currently support): https://github.com/ekalinin/sitemap.js/releases/tag/9.0.0

However I think it's possible to upgrade to v8

I doubt any of the security issue fixed in the lib really affect anyone, so it's not a priority to do this upgrade. We'll do this for Docusaurus v4.