Disable CI for GCC 5 and 6

They can't compile the new constexpr decomposer because
`ITransientExpression` has a defaulted virtual destructor. We could
instead drop the virtual destructor - it only exists to silence
static analysis tools complaining about having virtual functions
without virtual destructor - but GCC 5 and 6 are positively ancient,
so we drop them instead.

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

@@ -9,8 +9,6 @@ jobs:
     strategy:
       matrix:
         cxx:
-          - g++-5
-          - g++-6
           - g++-7
           - g++-8
           - g++-9
@@ -23,10 +21,6 @@ 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