Drop Python 3.6 (and maybe 3.7) support

[eagleoflqj]

Related: #23378
It's unclear to me which version of Python is supported now and I see compatibility code for Python 3.6 when browsing code.
From #23363 (comment) min node version comes from Ubuntu 20.04, which provides Python 3.8.2.
Python 3.6 itself reached EOL on 2021-12-23, and even 3.8 reached EOL on 2024-10-07.
As the next emscripten release will be 4.0, I'm proposing drop Python 3.6 and 3.7 support as part of it, and set up a support policy if not exist.

[eagleoflqj]

Dup #23387