cmd/tailscaled,feature/conn25,feature/featuretags: add conn25 to featuretags

Package feature/conn25 is excludeable from a build via the featuretag.
Test it is excluded for minimal builds.

Updates #12614

Signed-off-by: Fran Bull <fran@tailscale.com>

feature/conn25/conn25.go

@@ -2,6 +2,9 @@
 // SPDX-License-Identifier: BSD-3-Clause
 
 // Package conn25 registers the conn25 feature and implements its associated ipnext.Extension.
+// conn25 will be an app connector like feature that routes traffic for configured domains via
+// connector devices and avoids the "too many routes" pitfall of app connector. It is currently
+// (2026-02-04) some peer API routes for clients to tell connectors about their desired routing.
 package conn25
 
 import (