Update truss examples to not use py38.

[squidarth]

We incremented the minimum python version for truss to 3.9. A bunch of our examples were still using py38.

Testing

I deployed the llama-2-7b-chat model successfully.

Follow-up

• Redeploy the model library model
• Add better validations to the truss config
• Remove python_version field from all of the truss examples (with exceptions for models that need to actually be pinned to specific versions)