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.

Dockerfile.cross

@@ -16,18 +16,6 @@ RUN apt-get update -qq && \
     apt-get clean -y && \
     rm -rf /var/lib/apt/lists/*
 
-# Install Autoconf 2.71. Ubuntu 20.04 comes with Autoconf 2.69, but
-# libffi 3.4.3 or newer requres Autoconf 2.71.
-RUN curl -LO https://ftp.gnu.org/gnu/autoconf/autoconf-2.71.tar.xz && \
-    tar -Jxvf autoconf-*.tar.xz && \
-    rm autoconf-*.tar.xz && \
-    cd autoconf-* && \
-    ./configure --prefix=/opt/autotools && \
-    make -j$(nproc) install && \
-    rm -rf autoconf-*
-
-ENV PATH=/opt/autotools/bin:$PATH
-
 COPY build-python.sh .
 
 ARG WITH_PYTHON