[CI] Remove deprecated Ubuntu 18.04 builds.

Author: mjpt777

.github/workflows/ci.yml

@@ -166,12 +166,11 @@ jobs:
           key: ${{ runner.os }}-gradle-wrapper-${{ hashFiles('**/gradle/wrapper/gradle-wrapper.properties') }}
       - name: Install compiler
         run: |
-          curl --retry ${INSTALL_COMPILER_RETRIES} -L https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-          echo 'Acquire::Retries "${INSTALL_COMPILER_RETRIES}";' | sudo tee -a /etc/apt/apt.conf.d/99retries
+          curl -L https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
           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 }} libsparsehash-dev libidn11
-          curl --retry ${INSTALL_COMPILER_RETRIES} -O -L https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/cmake-${CMAKE_VERSION}-Linux-x86_64.tar.gz
+          curl -O -L https://github.com/Kitware/CMake/releases/download/v${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
       - name: Build