Upgrade to pydantic 2

[ghidalgo3]

Description

Dependency constraint upgrade and code changes to support Pydantic > 2. Summary:

1. In all pyproject.toml files, the pydantic dependency changed from pydantic>=1.9,<2.0.0 to pydantic>=2.0.0 and a new dependency on pydantic-settings added because BaseSettings now comes from that package.
2. requirements.txt were regenerated
3. parse_obj -> model_validate
4. dict(**kwargs) -> model_dump(**kwargs)
5. class Config: -> model_config : dict
6. kwarg change: Field(const=True) -> Field(frozen=True)
7. @validator to either @field_validator or @model_validator
8. Changed YAML sourced settings to use the settings_customise_sources Pydantic mechanism.
9. pydantic.error_wrappers.ErrorList -> pydantic_core.ErrorDetails
10. One instance of changing Union[...] to Annotated[Union[...], Discriminator] to get that discriminated union goodness.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Checklist:

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• I have performed a self-review
• Changelog has been updated
• Documentation has been updated
• Unit tests pass locally (./scripts/test)
• Code is linted and styled (./scripts/format)