util/backoff: rename logtail/backoff package to util/backoff

It has nothing to do with logtail and is confusing named like that.

Updates #cleanup
Updates #17323

Change-Id: Idd34587ba186a2416725f72ffc4c5778b0b9db4a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

prober/prober.go

@@ -317,7 +317,7 @@ func (p *Probe) loop() {
 			p.run()
 			// Wait and then retry if probe fails. We use the inverse of the
 			// configured negative interval as our sleep period.
-			// TODO(percy):implement exponential backoff, possibly using logtail/backoff.
+			// TODO(percy):implement exponential backoff, possibly using util/backoff.
 			select {
 			case <-time.After(-1 * p.interval):
 				p.run()