Update Security String to 2021-09-05

Bug: 192405736
Change-Id: Icea699ec7f82744ff0a6dca90068b2737129778a
(cherry picked from commit cfd4896dc2da5f40c9e2eb33424d9917f8455edc)
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index f900f89..4bc4498 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -240,7 +240,7 @@
     #  It must be of the form "YYYY-MM-DD" on production devices.
     #  It must match one of the Android Security Patch Level strings of the Public Security Bulletins.
     #  If there is no $PLATFORM_SECURITY_PATCH set, keep it empty.
-      PLATFORM_SECURITY_PATCH := 2021-08-05
+      PLATFORM_SECURITY_PATCH := 2021-09-05
 endif
 .KATI_READONLY := PLATFORM_SECURITY_PATCH