Drop support for Python 3.7

[edgarrmondragon]

According to https://pypistats.org/packages/meltano-edk, a significant percentage of installations of this library is still done on Python 3.7 so we may to wait a bit to drop support.

[WillDaSilva]

@edgarrmondragon If we release a new version without support for Python 3.7, nothing stops users from continuing to use the old version. I think we should move ahead with Pydantic 2 support, and drop Python 3.7 as required.