upath.registry: don't rely on fsspec.registry.available_protocols

[ap--]

We should take better care of our minimum version boundaries. (should be easy too... We have one dependency 😅 )

For now, restore compatibility with fsspec<2022.03.0.

A few things to note though, when running with an older fsspec:

• gcsfs mkdir tests fail with gcsfs<2022.03.0
• webdav protocol is not known in fsspec<2022.05.0

Addresses #138