[ci] remove support for gcc-6

.github/workflows/build.yml

@@ -74,9 +74,6 @@ jobs:
       fail-fast: false
       matrix:
         config:
-          - name: "Build Compatibility: GCC-6 (Ubuntu 20.04)"
-            cc: gcc-6
-            cxx: g++-6
           - name: "Build Compatibility: GCC-7 (Ubuntu 20.04)"
             cc: gcc-7
             cxx: g++-7

.github/workflows/docker.yml

@@ -64,7 +64,6 @@ jobs:
     strategy:
       matrix:
         compiler:
-          - gcc-6
           - gcc-7
           - gcc-8
           - gcc-9

.github/workflows/install_dependencies_build.sh

@@ -45,8 +45,6 @@ sudo apt-get install -y \
     zip \
     swig \
     expect \
-    g++-6 \
-    gcc-6 \
     g++-7 \
     gcc-7 \
     g++-8 \