[xml] Resuming maintenance

[Nick Wellnhofer <wellnhofer aevum de>]

Hello,

Thanks to a donation from Google, I'm able to resume maintenance of libxml2 
(and libxslt) for the remainder of 2022.

My immediate plans are:

- Make a bug fix release fixing many regressions.
- Establish a new release schedule, possibly with multiple branches being
  maintained.
- Move releases from the old FTP server to GNOME's Gitlab infrastructure.
- Move documentation to GNOME infrastructure.
- Set up an official way to sponsor libxml2 maintainers.

In the future I'll focus less on security improvements and more on typical 
maintenance duties like bug fixes and modernizing the code base in a few ways.

Thanks (again) to Google for making this possible.

Nick