all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/conn25/conn25_test.go

@@ -115,7 +115,7 @@ func TestHandleConnectorTransitIPRequestMultipleTIP(t *testing.T) {
 		t.Fatalf("n TransitIPs in response: %d, want 3", len(resp.TransitIPs))
 	}
 
-	for i := 0; i < 3; i++ {
+	for i := range 3 {
 		got := resp.TransitIPs[i].Code
 		if got != TransitIPResponseCode(0) {
 			t.Fatalf("i=%d TransitIP Code: %d, want 0", i, got)