DOC: Update Python version support
The supported versions in https://pandas.pydata.org/docs/getting_started/install.html#python-version-support are hard-coded and the reasoning/criteria behind that is ambiguous.

The policy in the [version policy](https://pandas.pydata.org/docs/development/policies.html#python-support) document is clearer, so I just copied it.
/preview Website preview of this PR available at: https://pandas.pydata.org/preview/pandas-dev/pandas/58680/ Thanks @Aloqeely 