docker: Bump the base images to Ubuntu 22.04

This raises the versions of bundled software, providing e.g.
autoconf 2.71, which is needed for Python 3.12.

As the releases are built (without a surrounding extra container)
on GitHub Actions these days, we don't need to hold the docker
images back to an old version for the sake of compatibility;
instead, updating to a newer version provides more modern tools
for users of the docker image.

release.sh

@@ -29,7 +29,7 @@ fi
 
 time docker build -f Dockerfile . -t mstorsjo/llvm-mingw:latest -t mstorsjo/llvm-mingw:$TAG
 
-DISTRO=ubuntu-20.04-$(uname -m)
+DISTRO=ubuntu-22.04-$(uname -m)
 docker run --rm mstorsjo/llvm-mingw:latest sh -c "cd /opt && mv llvm-mingw llvm-mingw-$TAG-ucrt-$DISTRO && tar -Jcvf - --format=ustar --numeric-owner --owner=0 --group=0 llvm-mingw-$TAG-ucrt-$DISTRO" > llvm-mingw-$TAG-ucrt-$DISTRO.tar.xz
 
 if [ -n "$NATIVEONLY" ]; then