Remove Python 3.8 support

[elliotgunton]

Python 3.8 is currently in security-fixes only, and reaches end of life later this year (2024-10) - we should drop support for 3.8, which will help clean up the repo.

List of tasks:

• Update pyproject.toml - remove 3.8 PyPi classifier, set python >=3.9, run poetry lock
• Remove from cicd.yaml, use 3.9 in pre-release.yaml, publish.yml
• Remove files added for backwards compat (unparse.py, graphlib.py, possibly some others)
• Use 3.9 for generating test data (update Makefile target)
• Use 3.9 for generating models using datamodel-codegen
• Use 3.9 as default global_config image and regenerate examples

[flaviuvadan]

I think this will also close #584 as a "not fix" (unfortunately :( )