cmake: Use C17 only with CMake 3.21 or above

[umireon]

Description

C17 is not supported until CMake 3.21 and C17 must be used with CMake 3.21 or above. https://cmake.org/cmake/help/latest/prop_tgt/C_STANDARD.html

Motivation and Context

CMake 3.20 is provided on openSUSE Leap 15.5 and the plugin building with CMake commands fails with the message that CMake does not understand the C standard of C17. I need this bug to be fixed to build on openSUSE Leap.

How Has This Been Tested?

This change is tested in our plugin. https://github.com/royshil/obs-backgroundremoval/pull/430

The building log of our plugin with this change is available: https://pmbs.links2linux.org/package/live_build_log/home:umireon/obs-backgroundremoval/15.5/x86_64

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.

[umireon]

@RytoEX I've fixed the commit message.

[PatTheMav]

Should be fine, we expect more recent versions on Windows and macOS anyway.