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>

cmd/tailscaled/deps_test.go

@@ -293,6 +293,7 @@ func TestMinTailscaledWithCLI(t *testing.T) {
 			"github.com/hdevalence/ed25519consensus": "unexpected ed25519consensus dep",
 			"tailscale.com/clientupdate/distsign":    "unexpected distsign dep",
 			"archive/tar":                            "unexpected archive/tar dep",
+			"tailscale.com/feature/conn25":           "unexpected conn25 dep",
 		},
 	}.Check(t)
 }