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>

cmd/k8s-operator/tsrecorder_specs.go

@@ -7,6 +7,7 @@ package main
 
 import (
 	"fmt"
+	"maps"
 
 	appsv1 "k8s.io/api/apps/v1"
 	corev1 "k8s.io/api/core/v1"
@@ -312,9 +313,7 @@ func tsrEnv(tsr *tsapi.Recorder, loginServer string) []corev1.EnvVar {
 
 func tsrLabels(app, instance string, customLabels map[string]string) map[string]string {
 	labels := make(map[string]string, len(customLabels)+3)
-	for k, v := range customLabels {
-		labels[k] = v
-	}
+	maps.Copy(labels, customLabels)
 
 	// ref: https://kubernetes.io/docs/concepts/overview/working-with-objects/common-labels/
 	labels["app.kubernetes.io/name"] = app