build: min version for cassandra and nfs tests is k8s 1.16

The minimum K8s version for v1 CRDs is 1.16. Since support for
v1beta1 cassandra and nfs was removed and only v1 is supported,
the CI will now only run back to 1.16 for those providers.

Signed-off-by: Travis Nielsen <tnielsen@redhat.com>

.github/workflows/integration-tests-on-release.yaml

@@ -275,7 +275,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        kubernetes-versions : ['v1.15.12','v1.18.15','v1.20.5','v1.21.0']
+        kubernetes-versions : ['v1.16.15','v1.18.15','v1.20.5','v1.21.0']
     steps:
     - name: checkout
       uses: actions/checkout@v2
@@ -326,7 +326,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        kubernetes-versions : ['v1.15.12','v1.18.15','v1.20.5','v1.21.0']
+        kubernetes-versions : ['v1.16.15','v1.18.15','v1.20.5','v1.21.0']
     steps:
     - name: checkout
       uses: actions/checkout@v2