go.mod: deprecate golang.org/x/exp/vulndb

The latest version of govulncheck and the vulncheck API now live at
golang.org/x/vuln.

This module is now deprecated.

Change-Id: If10c44bf7c9d2f5c473a318714ae87e013561c8c
Reviewed-on: https://go-review.googlesource.com/c/exp/+/395454
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/vulndb/go.mod b/vulndb/go.mod
index 214b441..4c7d6fc 100644
--- a/vulndb/go.mod
+++ b/vulndb/go.mod
@@ -1,3 +1,4 @@
+// Deprecated: use golang.org/x/vuln instead.
 module golang.org/x/exp/vulndb
 
 go 1.17