Update libxml2 from 2.10.4 to 2.12.6 #6

Job	Run time
jruby-package	1m 39s
cruby-native-package (aarch64-linux)	2m 49s
cruby-package	2m 22s
cruby-native-package (x64-mingw-ucrt)	3m 30s
cruby-native-package (x64-mingw32)	3m 28s
cruby-native-package (x86-linux)	2m 18s
cruby-native-package (x86_64-darwin)	3m 26s
cruby-native-package (x86_64-linux)	2m 24s
jruby-install (jruby-9.3)	1m 5s
jruby-install (jruby-head)	1m 6s
cruby-windows-install-ucrt (enable)	43s
cruby-linux-install (enable, 2.6)	54s
cruby-windows-install-ucrt (disable)	43s
cruby-windows-install (enable)	44s
cruby-linux-install (enable, 2.7)	54s
cruby-osx-install (enable)	43s
cruby-windows-install (disable)	43s
cruby-linux-install (enable, 3.0)	33s
cruby-osx-install (disable)	54s
cruby-linux-install (enable, 3.1)	40s
cruby-linux-install (enable, head)	33s
cruby-linux-install (disable, 2.6)	55s
cruby-linux-install (disable, 2.7)	54s
cruby-linux-install (disable, 3.0)	55s
cruby-linux-install (disable, 3.1)	53s
cruby-linux-install (disable, head)	53s
cruby-aarch64-linux-install	1s
cruby-x86_64-linux-install	1s
cruby-x86_64-darwin-install	1s
cruby-x86_64-musl-install	1s
cruby-x86-linux-install	1s
cruby-x64-mingw32-install	1s
cruby-x64-mingw-ucrt-install	1s
	36m 48s

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