Reintroduce --warnings-as-errors in CI

What changed?
============

Commit 8385f07 removed our
`--warnings-as-errors` flag in CI while we had internal deprecation
warnings. Now that we've removed the deprecated functions, we
reintroduce the flag.

Author: germsvel

.github/workflows/ci.yml

@@ -64,7 +64,7 @@ jobs:
       run: mix deps.get
 
     - name: Compiles without warnings
-      run: mix compile
+      run: mix compile --warnings-as-errors
 
     - name: Check Formatting
       run: mix format --check-formatted