ApacheHttp2Transport connection did not return to the pool when it is failed

Firebase Admin Java SDK v9.6.0

```
FirebaseOptions.Builder builder = FirebaseOptions.builder()
...
.setHttpTransport(new ApacheHttp2Transport(httpClient));
```

```
ApiFuture futures = firebaseMessaging.sendEachAsync(messages, dryRun);
BatchResponse response;
try {
response = futures.get(TIME_OUT_MS, TimeUnit.MILLISECONDS);
} catch (TimeoutException e) {
...
}
```

```
final PoolStats total = poolingAsyncClientConnectionManager.getTotalStats();
return String.format("Available=%d, leased=%d, pending=%d, max=%d",
total.getAvailable(),
total.getLeased(),
total.getPending(),
total.getMax());
```

Observation:
Under very busy network, FCM backend may return `Unknown error while making a remote service call: Write Timeout` or other errors.

Log writing (successful or failed response) become slow and slower. Afterward, all pending requests were timed out at ` response = futures.get(TIME_OUT_MS, TimeUnit.MILLISECONDS);`

```
When the program just started:
Available=95, leased=5, pending=0, max=200
```
```
After some times:
Available=0, leased=0, pending=0, max=200
```

-----------------------------------------------------------------------------------------------------------------------
Update:

When using `Google APIs Transports`:
There is `disconnect` implementation in [google-http-java-client](https://github.com/googleapis/google-http-java-client/blob/main/google-http-client/src/main/java/com/google/api/client/http/apache/ApacheHttpResponse.java#L118)

However, when using `ApacheHttp2Transport`:
There is no such implementation in [ApacheHttp2Response ](https://github.com/firebase/firebase-admin-java/blob/master/src/main/java/com/google/firebase/internal/ApacheHttp2Response.java)
when it is [disconnected](https://github.com/firebase/firebase-admin-java/blob/master/src/main/java/com/google/firebase/internal/ErrorHandlingHttpClient.java#L113), no matter the response is successfully or failed.