Enable Ruby 3 for all users

[Bo98]

This will:

• Make all macOS users use Portable Ruby (subsequent cleanup of system Ruby logic will follow later).
• Remove availability of Portable Ruby 2.6
• Allow system Ruby 3.1 to be used on Linux

This will not:

• Require system Ruby users on Linux to change their Ruby from 2.6. That is a breaking change and will follow after 4.1.23, which will likely be the final 4.1.x.

This will apply to dev-cmd users immediately once this is merged and all other users when 4.1.23 is tagged.

Draft because #16267 needs to be merged first. Consider this otherwise ready for review.

[MikeMcQuaid]

Looks good to me, thanks!