Drop greater than or equal to python 3.9 checks in type serialization

[sjrl]

As a follow up to #8971 and #8894 we should drop the python 3.8 specific behavior used here

haystack/haystack/utils/type_serialization.py

Line 125 in c4fafd9

if sys.version_info >= (3, 9) or repr(main_type).startswith("typing."):

This will help to simplify the code and will eventually be needed to tackle the typing library deprecation. E.g. https://stackoverflow.com/questions/66738753/python-typing-deprecation