.github: change label for vulndb and vulndb issues

.github: change label for vulndb and vulndb issues

Change-Id: Ie384bfb366abc4005e9333b24dbbb70d66b74c0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/427956
Reviewed-by: Tatiana Bradley <tatiana@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Julie Qiu <julieqiu@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Julie Qiu <julieqiu@google.com>