Migrate to pydantic v2

[AdeelH]

Overview

This PR migrates the code from using pydantic v1 to v2 and updates the version requirement to >=2.

The main changes are:

• Handling of serialization. The default serialization behavior has changed in v2. See https://docs.pydantic.dev/latest/concepts/serialization/#serializing-with-duck-typing. We use .model_dump(serialize_as_any=True) to get the old behavior.
• Replacement of validator and root_validator with field_validator and model_validator respectively.

Checklist

• [x] Added unit tests, if applicable
• [x] Updated documentation, if applicable
• [ ] Added needs-backport label if the change should be back-ported to the previous release
• [x] PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

See updated unit tests.

Closes #2034

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.38095% with 16 lines in your changes missing coverage. Please review.

Project coverage is 90.15%. Comparing base (b74e30e) to head (be455b9).

Files	Patch %	Lines
...ner/rastervision/pytorch_learner/learner_config.py	86.56%	9 Missing :warning:
..._pipeline/rastervision/pipeline/pipeline_config.py	66.66%	4 Missing :warning:
...tervision_pipeline/rastervision/pipeline/config.py	89.47%	2 Missing :warning:
...n/core/rv_pipeline/semantic_segmentation_config.py	92.85%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2178 +/- ## ========================================== + Coverage 90.10% 90.15% +0.04% ========================================== Files 197 198 +1 Lines 9874 9879 +5 ========================================== + Hits 8897 8906 +9 + Misses 977 973 -4 ```

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