Readme: Clarify GCC c++17 support

[ptheywood]

Our readme currently states we require GCC >= 8.1 for c++17 support (for mostly).

In pratice, GCC 8.1 is not fully C++17 compliant, as it does not implement the parallel algorithms TS (i.e. std::reduce, std::inclusive_scan).

Currently, this is not a problem for FLAME GPU itself, as we do not use this part of stdlib, but we may introduce it in the future, and CI will not trivially include a GCC 8 build once Ubuntu 20.04 is removerd from github actions (probably in 2025).

We should probably clarify in the readme that we require GCC >= 9 for c++17, but that GCC 8 may work (with a number of caveats).