go.mod: use go 1.18

Change-Id: I9ba8b1ce790de77d975999d486250b680ae46d1b
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/393775
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/go.mod b/go.mod
index 40c1311..8284c16 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module golang.org/x/vulndb
 
-go 1.17
+go 1.18
 
 require (
 	golang.org/x/vuln v0.0.0-20211221130724-9d39a965865f