Update CI to use Elixir 1.13.4/OTP 25.2

Author: paulgoetze

.github/workflows/elixir.yml

@@ -15,8 +15,8 @@ jobs:
     - name: Setup elixir
       uses: actions/setup-elixir@v1
       with:
-        elixir-version: 1.9.4 # Define the elixir version [required]
-        otp-version: 22.2 # Define the OTP version [required]
+        elixir-version: 1.13.4 # Define the elixir version [required]
+        otp-version: 25.2 # Define the OTP version [required]
     - name: Check formatting
       run: mix format --check-formatted