fix: Pydantic, Pypgstac and Python Upgrades

[ividito]

Issue

#435
#436
#437

What?

• Upgraded Pydantic to version 2, which required several changes to validators. In particular, most decorators changed, and all Optional values must explicitly default to None.
• Upgraded pypgstac to 0.8.5
• Upgraded Python runtimes to avoid errors installing Pydantic on earlier Python versions

Testing?

• Tested manually locally via docker compose, and tested CDK build.