Only check formatting on Elixir 1.15

Author: axelson

.github/workflows/ci.yml

@@ -51,6 +51,7 @@ jobs:
       run: mix compile --warnings-as-errors
     - name: Check formatting
       run: mix format --check-formatted
+      if: startsWith(matrix.elixir, '1.15')
     - name: Run Tests
       run: mix test
     - name: Run credo