Update Elixir versions in CI workflow

Author: jonasschmidt

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
         include:
           - elixir: 1.15.8
             otp: 24.3.4.17
-          - elixir: 1.17.3
+          - elixir: 1.18.1
             otp: 27.2
     env:
       MIX_ENV: test