Set snap as stable (#805)

Author: lucacome

.goreleaser.yml

@@ -116,7 +116,7 @@ snapcrafts:
     description: |
       This package contains software that integrates NGINX Plus
       with AWS Auto Scaling groups and Azure Virtual Machine Scale Set
-    grade: devel
+    grade: stable
     confinement: strict
     publish: true
     license: BSD-2-Clause