[xml] Release of libxml2 2.10.2

[Nick Wellnhofer <wellnhofer aevum de>]

Version 2.10.2 of libxml2 is available at:

    https://download.gnome.org/sources/libxml2/2.10/

This should really fix the build with Python 3.10.

### Improvements

- Remove set-but-unused variable in xmlXPathScanName
- Silence -Warray-bounds warning

### Build system

- build: require automake-1.16.3 or later (Xi Ruoyao)
- Remove generated files from distribution

### Test suite

- Don't create missing.xml when running testapi

Nick