Update compiler versions

[dzenanz]

I think these are out of date:

ITKSoftwareGuide/SoftwareGuide/Latex/Introduction/Installation.tex

Lines 36 to 38 in 520cac8

	\item GCC 7 and newer
	\item Visual Studio 2017 version 15.7 and newer
	\item Xcode 10.0 (Apple LLVM Clang version 10.0.0) and newer

I think we have needed VS2019 for a while now.

[thewtex]

We have a list here: https://docs.itk.org/en/latest/supported_compilers.html