Add back g++ 5 and 6 to the CI builds

Previously these were removed in bbba3d8a068aeb5ec677db30947fe97fe047a2ba,
to allow Decomposer changes that were not compatible with old
GCCs. However, the compatibility was restored in
459ac8562b4f4b23280003beabeca910e823627a, and so we can restore
the builds again.

.github/workflows/linux-simple-builds.yml

@@ -9,6 +9,8 @@ jobs:
     strategy:
       matrix:
         cxx:
+          - g++-5
+          - g++-6
           - g++-7
           - g++-8
           - g++-9
@@ -21,6 +23,10 @@ jobs:
         build_type: [Debug, Release]
         std: [14]
         include:
+          - cxx: g++-5
+            other_pkgs: g++-5
+          - cxx: g++-6
+            other_pkgs: g++-6
           - cxx: g++-7
             other_pkgs: g++-7
           - cxx: g++-8