re-use transport for AdminInfo() call (#14571)

avoids creating new transport for each `isServerResolvable`
request, instead re-use the available global transport and do
not try to forcibly close connections to avoid TIME_WAIT
build upon large clusters.

Never use httpClient.CloseIdleConnections() since that can have
a drastic effect on existing connections on the transport pool.

Remove it everywhere.

internal/config/identity/openid/jwt.go

@@ -471,7 +471,6 @@ func parseDiscoveryDoc(u *xnet.URL, transport *http.Transport, closeRespFn func(
 	}
 	resp, err := clnt.Do(req)
 	if err != nil {
-		clnt.CloseIdleConnections()
 		return d, err
 	}
 	defer closeRespFn(resp.Body)