fix: bump Python base image and update deprecated PyGithub usage

[pb01ka]

Summary

Fixes conda-forge/admin-requests#2120

• Bump the Docker base image from python:3.8-slim to python:3.12-slim. Python 3.8 is EOL, and cryptography has announced it is dropping 3.8 support, which would break this image's build outright.
• Pin pygithub to 2.8.1 instead of installing unpinned, so future major-version upgrades are deliberate rather than silent.
• Update github.Github(os.environ["GITHUB_TOKEN"]) in turnstyle.py to the non-deprecated github.Github(auth=github.Auth.Token(...)) form required by PyGithub 2.x.

Together these resolve the two deprecation warnings currently surfacing in CI:

CryptographyDeprecationWarning: Python 3.8 is no longer supported by the Python core team...
DeprecationWarning: Argument login_or_token is deprecated, please use auth=github.Auth.Token(...) instead

[pb01ka]

@beckermr Please take a look. This is my first PR in this repository, so it needs an approval to trigger the CI. TY.

[pb01ka]

That was quick. TYSM @beckermr. 🥳 ❤️