[BE] Apply pep604 and turn on flake8 rule so no regressions

[Lucaskabela]

Since python>=3.10 is defined in the pyproject.toml, we can enable pep604 (in python >=3.10)

Why?

1. Fewer imports making code simpler
2. Standardization - prior to this change, we could have Optional or the | None intermixed in the codebase
3. Future proof for direction of python type system

Test

Ran

 python -m flake8 --pep604-activation=always .

Explicitly

[tianyu-l]

Thanks, this looks quite useful! Would prefer landing this after #2386

[Lucaskabela]

Review #2411 first then will rebase and fix merge conflicts