add additional reserved team name (#13606)

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

model/utils.go

@@ -376,6 +376,7 @@ var reservedName = []string{
 	"oauth",
 	"error",
 	"help",
+	"plugins",
 }
 
 func IsValidChannelIdentifier(s string) bool {