go.mod: update to go1.16

Change-Id: I35e9c3b0f67711923d85fe221948fcb3abbb0a4f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/343939
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
diff --git a/go.mod b/go.mod
index 05a81bf..4d6aa18 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module golang.org/x/pkgsite
 
-go 1.15
+go 1.16
 
 require (
 	cloud.google.com/go v0.66.0