Remove k8s < 1.19 from nightly

Author: pleshakov

.github/workflows/nightly.yml

@@ -121,7 +121,7 @@ jobs:
     needs: [build, binary, unit-tests]
     strategy:
       matrix:
-        k8s: [1.21.1, 1.20.7, 1.19.11, 1.18.19, 1.17.17, 1.16.15]
+        k8s: [1.21.1, 1.20.7, 1.19.11]
     steps:
       - name: Checkout Repository
         uses: actions/checkout@v2