chore: Run test matrix on Elixir 1.20.x (#167)

The first RC of Elixir 1.20 has just been released.

Running the CI on that version will allow us to catch type errors
quickly and fix them before 1.20 is officially released.

Author: crbelaus

.github/workflows/elixir.yml

@@ -28,6 +28,8 @@ jobs:
             erlang: "27.x"
           - elixir: "1.19.x"
             erlang: "28.x"
+          - elixir: "latest"
+            erlang: "28.x"
     services:
       db:
         image: postgres:15
@@ -86,16 +88,16 @@ jobs:
         run: mix compile --force --warnings-as-errors
 
       - name: Run Tests - SQLite3
-        run: mix test
+        run: mix test --warnings-as-errors
         env:
           DB: sqlite
 
       - name: Run Tests - PostgreSQL
-        run: mix test
+        run: mix test --warnings-as-errors
         env:
           DB: postgres
 
       - name: Run Tests - MySQL/MariaDB
-        run: mix test
+        run: mix test --warnings-as-errors
         env:
           DB: mysql