All-Projects: Fix trailer list in validation regex

Adds security-medium and security-low to the list of valid trailers.

BUG=b/291595251

Change-Id: Ie7f0264a58569de653c253b1f44f4ce0111d2b5c
Reviewed-on: https://cos-review.googlesource.com/c/All-Projects/+/66195
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Arnav Kansal <rnv@google.com>
Tested-by: Kevin Berry <kpberry@google.com>