AST-1553 Remove IDG from Ubuntu 20 build; Add Ubuntu 24 build

• Since IDG no longer supports Ubuntu 20 (it uses CMake 3.17, while Ubuntu 20 has CMake 3.16), remove it from the Ubuntu 20 CI build.
• Add a basic Ubuntu 24 build on CI.

docker/ubuntu_20_04_base

+FROM ubuntu:20.04
+
+# TODO: needs to be bumped before next DP3 release
+ENV EVERYBEAM_VERSION=v0.5.8
+ENV AOFLAGGER_VERSION=b1256de90b00a5a83477274390decd6671cdcd38
+
+RUN export DEBIAN_FRONTEND="noninteractive" && \
+	apt-get update && \
+	apt-get install -y \
+	bison \
+	build-essential \
+	casacore-dev \
+	casacore-tools \
+	cmake \
+	doxygen \
+	flex \
+	gfortran \
+	git \
+	libarmadillo-dev \
+	libboost-date-time-dev \
+	libboost-filesystem-dev \
+	libboost-program-options-dev \
+	libboost-python-dev \
+	libboost-system-dev \
+	libboost-test-dev \
+	libcfitsio-dev \
+	libfftw3-dev \
+	libgtkmm-3.0-dev \
+	libhdf5-serial-dev \
+	liblua5.3-dev \
+	libpng-dev \
+	ninja-build \
+	pkg-config \
+	pybind11-dev \
+	python3-dev \
+	python3-numpy \
+	python3-pip \
+	wcslib-dev \
+	wget \
+	&& \
+	rm -rf /var/lib/apt/lists/*
+# Build aoflagger3
+RUN mkdir /aoflagger && cd /aoflagger \
+	&& git clone https://gitlab.com/aroffringa/aoflagger.git src \
+	&& ( cd src/ && git checkout ${AOFLAGGER_VERSION} ) \
+	&& mkdir build && cd build \
+	&& cmake -G Ninja -DCMAKE_INSTALL_PREFIX=/usr ../src \
+	&& ninja install \
+	&& cd / && rm -rf aoflagger
+# Build EveryBeam
+RUN mkdir /everybeam && cd /everybeam \
+	&& git clone https://git.astron.nl/RD/EveryBeam.git src \
+	&& ( cd src/ && git checkout ${EVERYBEAM_VERSION} ) \
+	&& mkdir build && cd build \
+	&& cmake -G Ninja -DCMAKE_INSTALL_PREFIX=/usr ../src -DPYTHON_EXECUTABLE=/usr/bin/python3 \
+	&& ninja install \
+	&& cd / && rm -rf everybeam
+# Install WSRT Measures (extra casacore data, for integration tests)
+# Note: The file on the ftp site is updated daily. When warnings regarding leap
+# seconds appear, ignore them or regenerate the docker image.
+RUN wget -nv -O /WSRT_Measures.ztar ftp://ftp.astron.nl/outgoing/Measures/WSRT_Measures.ztar \
+	&& cd /var/lib/casacore/data \
+	&& tar xfz /WSRT_Measures.ztar \
+	&& rm /WSRT_Measures.ztar
+# Install pip dependencies
+RUN pip3 install \
+	autosemver==0.5.5 \
+	gcovr \
+	h5py \
+	jsonschema2rst==0.1.0 \
+	sphinx \
+	sphinx-rtd-theme \
+	;