chore: fix Elixir 1.20 warnings

[martosaur]

💡 Motivation and Context

Elixir 1.20 released today with a ton of new compiler checks related to the type system. For posthog sdk, there was just one warning for an unreachable clause, so let's fix it.

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

[greptile-apps]

_{Reviews (1): Last reviewed commit: "chore: fix Elixir 1.20 warnings" | Re-trigger Greptile}

[turnipdabeets]

LGTM, worth updating the CI matrix in .github/workflows/ci.yml since it tops out at 1.19.x

[martosaur]

Update CI versions matrix to reflect official compatibility doc!