Add missing GCC version to settings.yml

[johnmcfarlane]

This version is already apt package, g++-10, in Ubuntu 20.04.

Changelog: Fix: Adds missing version GCC 10 to default settings.
Docs: conan-io/docs#1675

Fix #6910
Fix #6920

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[CLAassistant]

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[memsharded]

Lets release this for next 1.25.

Please @SSE4 , can you provide the default cppstd of this compiler version in the codebase?

[johnmcfarlane]

Lets release this for next 1.25.

Please @SSE4 , can you provide the default cppstd of this compiler version in the codebase?

99% certain it's 14. Trunk is 14 and when when I try the same test in an Ubuntu 20.04 docker container (johnmcfarlane/cnl_ci:gcc-10) it also reports 14.
Thanks

[SSE4]

I confirm it's C++14, thus no changes for cppstd_default are needed.

g++ -dM -E -x c++  /dev/null | grep __cplus
#define __cplusplus 201402L