app/appengine: hide and stop testing release-branch.go1.9

Per policy, we only support and test the past two releases
(currently Go 1.11 and Go 1.10).

Previously:

-	https://golang.org/cl/95555 removed Go 1.8.
-	https://golang.org/cl/77931 removed Go 1.7.
-	https://golang.org/cl/37466 removed Go 1.6.
-	https://golang.org/cl/31767 removed Go 1.5.
-	https://golang.org/cl/20843 removed Go 1.4.

Fixes golang/go#27541.

Change-Id: Ia42e9caae2bd6737ded22d871eeb24d00a06fa4a
Reviewed-on: https://go-review.googlesource.com/133895
Reviewed-by: Andrew Bonventre <[email protected]>

Author: dmitshur

app/appengine/dash.go

@@ -209,4 +209,5 @@ var hiddenBranches = map[string]bool{
 	"release-branch.go1.6": true,
 	"release-branch.go1.7": true,
 	"release-branch.go1.8": true,
+	"release-branch.go1.9": true,
 }