Add support for pydantic v2

[gotmax23]

As of 03a2882, we restricted pydantic to v1. Now that pydantic v2 is GA, we should fix any incompats, relax this pin, and make sure to test with both versions in CI.

[felixfontein]

The migration guide will be helpful: https://docs.pydantic.dev/2.0/migration/

[gotmax23]

This shouldn't be too difficult, but we'll have to silence some DeprecationWarnings while we're still supporting pydantic v1. I'm not sure what to do about

antsibull-core/src/antsibull_core/schemas/context.py

Line 63 in be14927

extra: ContextDict = ContextDict()

though. The way pydantic v2 handles extra arguments is a bit different.