cmd/gopherbot: add unreleased milestone to go.dev issues

This change updates gopherbot to add the Unreleased milestone for go.dev
issues if they do not yet have a milestone.

Tested locally with dry-run.

Updates golang/go#36749

Change-Id: I1fe0739d15961669950b8f65a8767a09c619943d
Reviewed-on: https://go-review.googlesource.com/c/build/+/217957
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/cmd/gopherbot/gopherbot.go b/cmd/gopherbot/gopherbot.go
index 18baaa9..f30a5f9 100644
--- a/cmd/gopherbot/gopherbot.go
+++ b/cmd/gopherbot/gopherbot.go
@@ -857,6 +857,9 @@
 		if strings.HasPrefix(gi.Title, "x/vgo") {
 			return b.setMilestone(ctx, gi, vgo)
 		}
+		if strings.HasPrefix(gi.Title, "go.dev:") {
+			return b.setMilestone(ctx, gi, unreleased)
+		}
 		return nil
 	})
 }