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>

tstest/integration/integration_test.go

@@ -1673,7 +1673,7 @@ func TestNetstackTCPLoopback(t *testing.T) {
 		defer lis.Close()
 
 		writeFn := func(conn net.Conn) error {
-			for i := 0; i < writeBufIterations; i++ {
+			for range writeBufIterations {
 				toWrite := make([]byte, writeBufSize)
 				var wrote int
 				for {