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

@@ -1,4 +1,4 @@
-FROM ubuntu:20.04
+FROM ubuntu:22.04
 
 RUN apt-get update -qq && \
     DEBIAN_FRONTEND="noninteractive" apt-get install -qqy --no-install-recommends \
@@ -8,16 +8,6 @@ RUN apt-get update -qq && \
     apt-get clean -y && \
     rm -rf /var/lib/apt/lists/*
 
-# Manually install a newer version of CMake; this is needed since building
-# LLVM requires CMake 3.20, while Ubuntu 20.04 ships with 3.16.3. If
-# updating to a newer distribution, this can be dropped.
-RUN cd /opt && \
-    curl -LO https://github.com/Kitware/CMake/releases/download/v3.26.4/cmake-3.26.4-Linux-$(uname -m).tar.gz && \
-    tar -zxf cmake-*.tar.gz && \
-    rm cmake-*.tar.gz && \
-    mv cmake-* cmake
-ENV PATH=/opt/cmake/bin:$PATH
-
 
 RUN git config --global user.name "LLVM MinGW" && \
     git config --global user.email root@localhost