Update ROOT to C++17 and drop default Werror

I was getting build errors with GCC9. As the CI is not so up to date anymore I suggest dropping Werror from the default build and only use it on the CI, as those warnings should not prevent users from building it on newer versions. The particular error was noexcept, but I did not manage to find the origin of it yet..

/home/koen/cern/workspace/allpix-squared/src/core/geometry/Detector.cpp:52:87: warning: noexcept-expression evaluates to ‘false’ because of a call to ‘constexpr std::pair<_T1, _T2>::pair() [with _U1 = double; _U2 = double; typename std::enable_if<std::__and_<std::__is_implicitly_default_constructible<_U1>, std::__is_implicitly_default_constructible<_U2> >::value, bool>::type <anonymous> = true; _T1 = double; _T2 = double]’ [-Wnoexcept]
   52 |     : name_(std::move(name)), position_(std::move(position)), orientation_(orientation) {}

I also updated the way for ROOT C++17 detection as the specific flag got removed it seems...