Update to C++20

[hoshinolina]

Description

Current CEF requires compiling as C++20.

Motivation and Context

Fails to build with CEF 145.

How Has This Been Tested?

Tested as local build, testing in Fedora shortly.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

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

[RytoEX]

Fix the merge conflict caused by #518.

[RytoEX]

Super nit, but looks fine otherwise.

[PatTheMav]

I'm on board with increasing the language level to C++20, however that change should affect all targets (and not just the Linux ones).

obs-browser is not a "standalone module" (I removed CMake code to that end IIRC) and thus should always inherit what the main project sets up as (via CMAKE_CXX_STANDARD).

And moving the project to C++20 is indeed in the works already, so the issue will be solved automatically through that.

[hoshinolina]

FWIW this patch is needed to build OBS with current CEF today. I'm not a CMake nor C++ expert, but building just the browser bits as C++20 (which is what this PR does) seems to work fine (and this patch is what Fedora is shipping).