Making GCC 9 the minimum required compiler target

[MikePopoloski]

I'm considering bumping the minimum required version of GCC to build the library (currently I support GCC 8). If anyone would be particularly affected by such a change please let me know here. GCC 9 was release over 3 years ago, so hopefully it's not too onerous. I'll leave this open for a week to see if anyone objects before moving forward.