This commit is to replace the existing but deprecating runtime key e.g.
global_downstream_max_connections by OverloadManager[^1] as suggested by
the below warning log.

```
level=warning msg="[Usage of the deprecated runtime key overload.global_downstream_max_connections, consider switching to `envoy.resource_monitors.downstream_connections` instead.This runtime key will be removed in future." subsys=envoy-main threadID=246
level=warning msg="[There is no configured limit to the number of allowed active downstream connections. Configure a limit in `envoy.resource_monitors.downstream_connections` resource monitor." subsys=envoy-main threadID=246
```

[^1]: https://www.envoyproxy.io/docs/envoy/latest/configuration/operations/overload_manager/overload_manager#limiting-active-connections

Signed-off-by: Tam Mach <tam.mach@cilium.io>