errors: make go directive explicit

In modern versions of the go command, if the go directive is missing,
go 1.16 is assumed. See https://go.dev/ref/mod#go-mod-file-go.

Add the go directive with 1.16 explicitly, otherwise running go mod tidy
(as done by the monthly tagging workflow) will initialize it with a much
higher version. This module holds experimental packages for an old draft
design, after all.

Change-Id: I0a31a56df624f6052249122c66153b91bb321a37
Reviewed-on: https://go-review.googlesource.com/c/exp/+/626216
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

errors/go.mod

@@ -1 +1,3 @@
 module golang.org/x/exp/errors
+
+go 1.16