go.mod: use Go 1.19

Change-Id: I4498bea570db18f0bc6c9aa9b942245bd43ce8b0
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/469775
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Julie Qiu <julieqiu@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
diff --git a/go.mod b/go.mod
index 3b167d2..d41aba6 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module golang.org/x/pkgsite-metrics
 
-go 1.18
+go 1.19
 
 require (
 	cloud.google.com/go v0.105.0