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>

wgengine/wgcfg/config_test.go

@@ -12,8 +12,7 @@ import (
 // that might get added in the future.
 func TestConfigEqual(t *testing.T) {
 	rt := reflect.TypeFor[Config]()
-	for i := range rt.NumField() {
-		sf := rt.Field(i)
+	for sf := range rt.Fields() {
 		switch sf.Name {
 		case "Name", "NodeID", "PrivateKey", "MTU", "Addresses", "DNS", "Peers",
 			"NetworkLogging":
@@ -28,8 +27,7 @@ func TestConfigEqual(t *testing.T) {
 // that might get added in the future.
 func TestPeerEqual(t *testing.T) {
 	rt := reflect.TypeFor[Peer]()
-	for i := range rt.NumField() {
-		sf := rt.Field(i)
+	for sf := range rt.Fields() {
 		switch sf.Name {
 		case "PublicKey", "DiscoKey", "AllowedIPs", "IsJailed",
 			"PersistentKeepalive", "V4MasqAddr", "V6MasqAddr", "WGEndpoint":