[release-branch.go1.15-security] go1.15.9

Change-Id: I99864b29d3bcbd07f3d8116d5997407152f1d462
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1014540
Reviewed-by: Katie Hockman <katiehockman@google.com>
diff --git a/VERSION b/VERSION
index 3a2ab1d..3c734ac 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-go1.15.8
\ No newline at end of file
+go1.15.9
\ No newline at end of file