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/magicsock/magicsock.go

@@ -1474,8 +1474,7 @@ func (c *Conn) sendUDPBatch(addr epAddr, buffs [][]byte, offset int) (sent bool,
 		err = c.pconn4.WriteWireGuardBatchTo(buffs, addr, offset)
 	}
 	if err != nil {
-		var errGSO neterror.ErrUDPGSODisabled
-		if errors.As(err, &errGSO) {
+		if errGSO, ok := errors.AsType[neterror.ErrUDPGSODisabled](err); ok {
 			c.logf("magicsock: %s", errGSO.Error())
 			err = errGSO.RetryErr
 		} else {

wgengine/magicsock/magicsock_test.go

@@ -2007,7 +2007,7 @@ func TestStressSetNetworkMap(t *testing.T) {
 
 	const iters = 1000 // approx 0.5s on an m1 mac
 	for range iters {
-		for j := 0; j < npeers; j++ {
+		for j := range npeers {
 			// Randomize which peers are present.
 			if prng.Int()&1 == 0 {
 				present[j] = !present[j]
@@ -2196,7 +2196,7 @@ func newWireguard(t *testing.T, uapi string, aips []netip.Prefix) (*device.Devic
 	if err != nil {
 		t.Fatal(err)
 	}
-	for _, line := range strings.Split(s, "\n") {
+	for line := range strings.SplitSeq(s, "\n") {
 		line = strings.TrimSpace(line)
 		if len(line) == 0 {
 			continue
@@ -4311,7 +4311,7 @@ func TestRotateDiscoKeyMultipleTimes(t *testing.T) {
 	keys := make([]key.DiscoPublic, 0, 5)
 	keys = append(keys, c.discoAtomic.Public())
 
-	for i := 0; i < 4; i++ {
+	for i := range 4 {
 		c.RotateDiscoKey()
 		newKey := c.discoAtomic.Public()