platformdirs >= 3.0.0 is ONLY required for macOS

[ThomasWaldmann]

Every other platform is fine with platformdirs 2.x.x also.

We need to add a comment about this to our setup.cfg so linux packagers don't run into unnecessary troubles.

Background:

• a major release number change indicates a breaking change
• the breakage introduced in platformdirs 3.0.0 is only because the macOS config directory changed (which was a bug fix, the old location was wrong), so this likely breaks upgrades on macOS.
• the config and other dirs for linux, bsd, windows did not change (AFAIK) by platformdirs 2.x.x -> 3.

https://github.com/platformdirs/platformdirs/blob/main/CHANGES.rst

[ThomasWaldmann]

The reason for borg to require platformdirs >=3.0.0 (on macOS) is to save macOS users from one future breakage by just starting with the new config dir.