Upgrade to Pydantic>=2

[pjbull]

With #277 we pin Pyandtic to <2.0.0. This is not ideal if people want to use zamba as a library package. We've got pretty extensive and complex use of validators in the codebase, so need to ensure that everything works as expected with the upgrade.

Here is the migration guide:

• https://docs.pydantic.dev/latest/migration/

There is an automated tool that is supposed to help do the migration for you:

• https://github.com/pydantic/bump-pydantic