CI: simplify Ubuntu package sources

.github/workflows/ci.yml

@@ -45,12 +45,9 @@ jobs:
     steps:
       - name: Install
         run: |
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ trusty main'
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ trusty universe'
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial main'
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial universe'
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic main'
-          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic universe'
+          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ trusty main universe'
+          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ xenial main universe'
+          sudo add-apt-repository -yn 'deb http://archive.ubuntu.com/ubuntu/ bionic main universe'
           sudo apt-get update
           sudo apt-get install -y gcc-${{ matrix.gcc }} g++-${{ matrix.gcc }}
       - name: Checkout