Update toolchain.py

[blackliner]

Changelog: Feature: Add support for CMAKE_CXX_STANDARD_REQUIRED in CMakeToolchain.
Docs: omit

Should this feature be controllable through a setting?

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[memsharded]

Looks good, I think it is ok to define it, as it is an "opt-in" defined by the definition of settings.compiler.cppstd, and if it is defined, it can be REQUIRED by cmake, to avoid fallback to previous standards.