Python 3.6 (probably <=3.8) deprecation in EB 5

[branfosj]

We should deprecate Python 3.6, and probably all Python <= 3.8, in EasyBuild 5.

Python 3.8 went EOL 7th October 2024.

While we still have one of the test bots running on Python 3.6 then we'll be probably be able to detect if we've introduced problems. But, once the test bots are all on a newer Python we'll be in the same situation as we've had with Python 2, such as adding f-strings in EB4.