OutlierDetection decrements ejectTimeBackoff too quickly

*Title*: OutlierDetection decrements `ejectTimeBackoff` too quickly

*Description*:

On every tick of the `intervalTimer` value of `ejectTimeBackoff` will be decremented for all not ejected hosts.
```c++
if (!host->healthFlagGet(Host::HealthFlag::FAILED_OUTLIER_CHECK)) {
// Node seems to be healthy and was not ejected since the last check.
if (monitor->ejectTimeBackoff() != 0) {
monitor->ejectTimeBackoff()--;
}
return;
}
```
But during the `interval` there could be no evidence that host actually became healthy.

*Repro steps*:

Let's say we have a host that constantly returns 500 status codes and we configured detector for 3 consecutive 5xx and the interval is equal to 1s:

1. Send 3 requests to the host, each request returns 500
2. OutlierDetection ejects this host for `baseEjectionTime * ejectTimeBackoff` where `ejectTimeBackoff` is equal 1
3. `ejectTimeBackoff++`
4. OutlierDetection unejects host after `baseEjectionTime`
5. After 1s `ejectTimeBackoff` is decremented
6. Send 3 requests to the host again, each returns 500
7. OutlierDetection ejects this host for `baseEjectionTime * ejectTimeBackoff` where `ejectTimeBackoff` is equal 1 again

Expected behaviour: step 7 should eject the host for `baseEjectionTime * 2`.
So at the step 5 OutlierDetection decrements `ejectTimeBackoff` without any knowledge if host recovered or still unhealthy.

*Conclusion*

This behaviour is confusing and not really documented properly. I think `ejectTimeBackoff` should be decremented only when there is convincing evidence that host is recovered.

xref: https://github.com/envoyproxy/envoy/pull/14235