[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