Factor out the CXX language standard in a separate const (#45374)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/45374

This change factors out the language standard in a separate constant so we can easily control it from a single place.

There are only 2 exception to this:
1. hermes-engine: the podspec is used in CI and it has no access to the rct_cxx_language_standard variable
2. Yoga: it can be used as a separate pod, outside of React Native, so it makes sense to leave it alone.

This change also fixes a problem where, in some setup, the language was set to C++14

## Changelog
[Internal] - Refactor Cxx language standard in a single constant

Reviewed By: dmytrorykun, blakef

Differential Revision: D59629061

fbshipit-source-id: 41eac64e47c14e239d8ee78bd88ea30af244d695