feat: remove support for Python 3.8, require 3.9 or higher (#3005)

Python 3.8 is End-of-Life (EOL) as of 2024-10 as stated in
https://devguide.python.org/versions/ and
https://peps.python.org/pep-0569/#lifespan

By dropping support for Python 3.8 and requiring Python 3.9 or higher
it allows python-gitlab to take advantage of new features in Python
3.9, which are documented at:
https://docs.python.org/3/whatsnew/3.9.html

Closes: #2968

BREAKING CHANGE: As of python-gitlab 5.0.0, Python 3.8 is no longer
supported. Python 3.9 or higher is required.