Add GCC 9 to default settings.yml - #5046

@Croydon please, add to the PR description:

Docs: conan-io/docs#1257

The default stdcpp version has to be adjusted if needed and also search for any other specific flag etc

I can't find yet information about the default -std parameter for gcc9. It is very likely it is going to be the same as gcc8 (gnu14), but I would prefer to have a confirmation before merging this. @Croydon @uilianries please try to gather that info.

The GNU team has not yet provided GCC 9 manual, only the changelog.

But reading the cxx-status pages:

C++14 Support in GCC

GCC has full support for the previous revision of the C++ standard, which was published in 2014.

This mode is the default in GCC 6.1 and above; it can be explicitly selected with the -std=c++14 command-line flag, or -std=gnu++14 to enable GNU extensions as well.

So it should be C++14 ...

To make sure, I've used conan new to create a simple test with conanio/gcc9:

#include <iostream>
int main() { std::cout << __cplusplus << std::endl; }

The result is: 201402

YES, I can confirm it is C++14 by default.