[CI] Remove deprecated Ubuntu 18.04 builds.

Author: mjpt777

.github/workflows/ci.yml

@@ -170,7 +170,7 @@ jobs:
           echo 'Acquire::Retries "${INSTALL_COMPILER_RETRIES}";' | sudo tee -a /etc/apt/apt.conf.d/99retries
           sudo add-apt-repository "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-${{ matrix.version }} main"
           sudo apt-get update
-          sudo apt-get install -y clang-${{ matrix.version }} libstdc++-7-dev libbsd-dev uuid-dev libidn11
+          sudo apt-get install -y clang-${{ matrix.version }} libsparsehash-dev libidn11
           curl --retry ${INSTALL_COMPILER_RETRIES} -O -L https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/cmake-${CMAKE_VERSION}-Linux-x86_64.tar.gz
           tar xf cmake-${CMAKE_VERSION}-Linux-x86_64.tar.gz
           echo "cmake-${CMAKE_VERSION}-Linux-x86_64/bin" >> $GITHUB_PATH