chore(README/maintainers): remove mxinden

[mxinden]

Remove myself from the list of rust-libp2p maintainers.

---

I am stepping down as a rust-libp2p maintainer. You can read more about it below.

https://max-inden.de/post/2024-02-29-stepping-down/

Couple of snippets:

I don’t see myself making major contributions to (rust-) libp2p in the near future and thus I am stepping down as a maintainer.

I enjoyed working with every maintainer, the larger libp2p team and the many contributors. Thank you! None of the above listed work would have been possible without you. None of it I could have achieved on my own. In the greater picture of the libp2p project, this list is negligeable, but probably fun for me to look back at in a couple of years from now. (👋 hi future Max.)

I will stay subscribed to the repository for now to answer questions. I assume some historic background will be useful from time to time. Feel free to tag me. In case my skillset is needed for larger work items, I am available on a project basis as a freelancer. Just send me a mail.

Related: #5007

[jxs]

Hi Max! Saddest approval ever 😞
As I told you I am deeply thankful to you for bringing me into libp2p and allowing me to work and maintain rust-libp2p,
and I aspire we revert this PR eventually future Max!