[ci] enable gcc-6 support

.github/workflows/build.yml

@@ -74,6 +74,9 @@ 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

@@ -64,13 +64,16 @@ jobs:
     strategy:
       matrix:
         compiler:
-          - gcc-5
           - gcc-6
           - gcc-7
           - gcc-8
           - gcc-9
+          - gcc-10
+          - gcc-11
           - clang-6.0
+          - clang-7
           - clang-8
+          - clang-10
     steps:
       - name: Checkout
         uses: actions/checkout@v2