Update CI runner from ubuntu-20.04 to ubuntu-latest (#128)

Ubuntu 20.04 runners are no longer available on GitHub Actions.
This updates the CI workflow to use ubuntu-latest, following the
pattern used in the successor project (sdk-python).

This change:
- Fixes "Requested labels: ubuntu-20.04" runner errors
- Aligns with GitHub's current runner offerings
- Matches the configuration in ReforgeHQ/sdk-python

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: jkebinger

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     env:
       MIX_ENV: test
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}