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

Previously these were removed in bbba3d8a06,
to allow Decomposer changes that were not compatible with old
GCCs. However, the compatibility was restored in
459ac8562b, 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