Drop support for older Pastas versions

[martinvonk]

We should think about how and when to deprecate older Pastas versions. The oldest version we support is Pastas v0.22.0. This version had Python v3.10 as highest dependency, which is now PastaStore's oldest supported Python version.

[dbrakenhoff]

I think we can just stop testing versions below Pastas 1.0 starting from the next release.

We've been supporting them forever, and I don't think too many people are relying on (or even using) Pastastore and really old versions of Pastas...