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>

control/controlhttp/http_test.go

@@ -814,8 +814,8 @@ func runDialPlanTest(t *testing.T, plan *tailcfg.ControlDialPlan, want []netip.A
 
 	// split on "|" first to remove memnet pipe suffix
 	addrPart := raddrStr
-	if idx := strings.Index(raddrStr, "|"); idx >= 0 {
-		addrPart = raddrStr[:idx]
+	if before, _, ok := strings.Cut(raddrStr, "|"); ok {
+		addrPart = before
 	}
 
 	host, _, err2 := net.SplitHostPort(addrPart)