use gcc 7 on ubuntu 18.04

Author: mayeut

.github/workflows/ci.yml

@@ -91,15 +91,6 @@ jobs:
         set -x
 
         case "${{ matrix.distro }}" in
-            ubuntu:18.04)
-                apt-get update
-                apt-get -y install software-properties-common
-                add-apt-repository -y ppa:ubuntu-toolchain-r/test
-                apt-get update
-                apt-get -y install automake g++-11 make
-                update-alternatives --install /usr/bin/c++ c++ $(which g++-11) 100
-                update-alternatives --install /usr/bin/cc cc $(which gcc-11) 100
-                ;;
             ubuntu*|debian*) apt-get update && apt-get -y install automake g++ make;;
         esac