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 {