feat: set CMAKE_CXX_STANDARD 20

[youtalk]

Description

This PR changes the CMAKE_CXX_STANDARD variable from 17 to 20.

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

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[xmfcx]

https://docs.ros.org/en/rolling/The-ROS2-Project/Contributing/Code-Style-Language-Versions.html#id2

Standard

Rolling targets C++17.

This is for ROS 2 rolling but I'm not sure if would cause an issue for us switching to C++20.

There is a length discussion here about it: https://discourse.ros.org/t/ros-2-and-c-20/34292/7 but I didn't go through all of it.

Is there a discussion post on this in Autoware side?

I was able to find this change through this PR:

• refactor(autoware_geography_utils): apply modern C++17 style autoware.core#109 referring to "use-c++20" branch here.

[youtalk]

I accidentally marked autowarefoundation/autoware.core#109 as Ready for review, but it is still in the draft stage. I am prototyping how much more refined it can be rewritten using C++20. Depending on the results, I will submit to the Discussions.

[youtalk]

Since we need to continue supporting some older OS versions, we will withdraw this for now.