CI: Remove Ubuntu 18 build (EOL)

ci/ubuntu-18.04/Dockerfile

@@ -1,48 +0,0 @@
-FROM ubuntu:18.04
-
-ENV DEBIAN_FRONTEND="noninteractive" TZ="America/Los_Angeles"
-
-# A version field to invalidate Cirrus's build cache when needed, as suggested in
-# https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20230801
-
-ENV CMAKE_DIR "/opt/cmake"
-ENV CMAKE_VERSION "3.19.1"
-ENV PATH "${CMAKE_DIR}/bin:${PATH}"
-
-RUN apt-get update && apt-get -y install \
-    bc \
-    bison \
-    bsdmainutils \
-    ccache \
-    clang-10 \
-    curl \
-    flex \
-    git \
-    lcov \
-    libkrb5-dev \
-    libmaxminddb-dev \
-    libpcap-dev \
-    libssl-dev \
-    make \
-    python3 \
-    python3-dev \
-    python3-pip\
-    ruby \
-    sqlite3 \
-    swig \
-    unzip \
-    wget \
-    zlib1g-dev \
-  && apt-get autoclean \
-  && rm -rf /var/lib/apt/lists/*
-
-# Recent CMake.
-RUN mkdir -p "${CMAKE_DIR}" \
-  && curl -sSL "https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/cmake-${CMAKE_VERSION}-Linux-x86_64.tar.gz" | tar xzf - -C "${CMAKE_DIR}" --strip-components 1
-
-RUN pip3 install websockets junit2html
-RUN gem install coveralls-lcov
-
-RUN update-alternatives --install /usr/bin/cc cc /usr/bin/clang-10 100
-RUN update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++-10 100