Add cache.c7gn high-bandwidth node types (#802)

Author: wata727

rules/utils.go

@@ -79,6 +79,11 @@ var validElastiCacheNodeTypes = map[string]bool{
 	"cache.r7g.16xlarge":  true,
 	"cache.c7gn.large":    true,
 	"cache.c7gn.xlarge":   true,
+	"cache.c7gn.2xlarge":  true,
+	"cache.c7gn.4xlarge":  true,
+	"cache.c7gn.8xlarge":  true,
+	"cache.c7gn.12xlarge": true,
+	"cache.c7gn.16xlarge": true,
 	"cache.m1.small":      true,
 	"cache.m1.medium":     true,
 	"cache.m1.large":      true,