Required compiler versions are out-of-date from Zeek documentation

[timwoj]

https://docs.zeek.org/en/master/install.html#required-dependencies states that we require GCC >= 8 and Clang >= 9. RequireCXX17.cmake requires GCC >= 7 and Clang >= 4, and should be updated to match the what our required dependencies are.

The Apple clang version can be updated to 11.4 as well, which is equivalent to Clang 9.0 according to https://en.wikipedia.org/wiki/Xcode#Xcode_11.0_-14.x(since_SwiftUI_framework)_2.