Update ci.yml

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{matrix.os}}
     strategy:
       matrix:
-        otp: [21.0, 22.0, 23.0, 24.0, 25.0]
+        otp: [22.0, 23.0, 24.0, 25.0]
         os: [ubuntu-latest]
     steps:
       - uses: actions/checkout@v2