Document minimum supported JupyterLab version

[JasonWeill]

The Jupyter Scheduler README says that it works with JupyterLab 3.0 or newer. Only 3.5.x and 3.6.x are still actively maintained, other than for security flaws and other high-severity issues. Issue #350 occurs with JupyterLab 3.4.4, but not with 3.5.3.

Document a minimum supported version of at least 3.5.x in the README and in the documentation.

[andrii-i]

• Taking in account, pinned issue JupyterLab 3, Jupyter Scheduler v1 branch — May 15, 2024 end of maintenance #509 and README update Clarify support lifecycle after JupyterLab 3 end of maintenance #508 clarify end of support / end of lifecycle for JupyterLab 3, this issue will become null at the end of the year as we are dropping support for JupyterLab 3.