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/stunstamp/stunstamp.go

@@ -889,8 +889,7 @@ func remoteWriteTimeSeries(client *remoteWriteClient, tsCh chan []prompb.TimeSer
 			reqCtx, cancel := context.WithTimeout(context.Background(), time.Second*30)
 			writeErr = client.write(reqCtx, ts)
 			cancel()
-			var re recoverableErr
-			recoverable := errors.As(writeErr, &re)
+			_, recoverable := errors.AsType[recoverableErr](writeErr)
 			if writeErr != nil {
 				log.Printf("remote write error(recoverable=%v): %v", recoverable, writeErr)
 			}