_content/doc/security/vuln: add WITHDRAWN to excluded reasons

- Updated docs to add WITHDRAWN as a valid reason a report is excluded.

Change-Id: I531d301553d8417eb82a796481be731b8cf8756e
Reviewed-on: https://go-review.googlesource.com/c/website/+/695156
Auto-Submit: Ethan Lee <ethanalee@google.com>
Reviewed-by: Markus Kusano <kusano@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: ethanalee-work

_content/doc/security/vuln/database.md

@@ -279,6 +279,7 @@ Reports may be excluded for these reasons:
   vulnerability entirely superseded by the report for A.
 - `NOT_A_VULNERABILITY`: While a CVE ID or GHSA has been assigned, there is no
   known vulnerability associated with it.
+- `WITHDRAWN`: The vulnerability has been withdrawn by its source.
 
 At the moment, excluded reports are not served via
 [vuln.go.dev](https://vuln.go.dev) API. However, if you have