Jupyter Scheduler 3.0.0 release plan

[andrii-i]

Issue to track and discuss features for the new major Jupyter Scheduler 3.0.0 release.

List of features

• Support multiple backends #597 (including backend for running Python files)
• Make Scheduler API calls async #598
• Consider upgrading to pydantic v2, removing pydantic v1 support #601
• Consider need to support legacy jobs in v3.0 #606

[andrii-i]

2.x branch https://github.com/jupyter-server/jupyter-scheduler/tree/2.x