Update pydantic to 2 to accommodate fastapi security updates

[loneil]

The dependabot security fix here https://github.com/bcgov/vc-authn-oidc/pull/412 should go in soon, but that has a conflict with the older version of Pydantic used.

Can see that in the test GHA step on that PR

/opt/hostedtoolcache/Python/3.11.7/x64/lib/python3.11/site-packages/pydantic/_migration.py:296: in wrapper
    raise PydanticImportError(
E   pydantic.errors.PydanticImportError: `BaseSettings` has been moved to the `pydantic-settings` package. See https://docs.pydantic.dev/2.6/migration/#basesettings-has-moved-to-pydantic-settings for more details.

~~Should be a simple fix to update and use pydantic-settings there~~ Update: there's a bunch more changes needed to accomodate Pydantic2