Adopt an fsspec dependency upgrade policy

[nicholasjng]

Dependency management is at the core of a successful open source project. Be too strict, and your package does not play nice with others. Be too lenient, and you might encounter sudden CI failures, bugs etc.

Our core dependency is fsspec, which itself has quite rapid development - for example, while the project is >4 years old, the ability to use transactions in context was only added in v2023.12.0, and remains as a patch in our repo specifically.

For this reason, I propose a policy of using an fsspec version no older than 3 months. I chose this duration because the release cadence seems to be one per month (except in summer), and three releases is a generous time to upgrade (contrast this to s3fs and gcsfs, which move in lockstep with fsspec, allowing no older versions at all!)

We should also only require >= qualifiers, since fsspec is at the same time stable enough that we can actually maintain that backward compatibility.

Opinions welcome!