[python] Drop Python 3.6 support

[StrikerRUS]

Python 3.6 is reaching its End of Life next week and no need to support it anymore.
https://devguide.python.org/#branchstatus

[jameslamb]

I support it! Especially since other dependencies of lightgbm have already dropped support for Python 3.6, like numpy (https://github.com/numpy/numpy/blob/0302205105161c6bbae5d45fe4e7a5a624f0dbf4/setup.py#L410) and pandas (https://github.com/pandas-dev/pandas/blob/47eb2198896852e17005629ed82c18940a05e975/setup.cfg#L40).

[StrikerRUS]

other dependencies of lightgbm have already dropped support for Python 3.6

They have already dropped even Python 3.7 (EoL 2023-06-27), interesting...

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.