Elasticsearch output: Documentation claims "upstream" support, but property is unknown

## Bug Report

**Describe the bug**

The 5.0 "Upstream servers" section documentation states the following (see [here](https://docs.fluentbit.io/manual/administration/configuring-fluent-bit/yaml/upstream-servers-section)):

> Examples of plugins that support this capability include [Forward](https://docs.fluentbit.io/manual/data-pipeline/outputs/forward) and [Elasticsearch](https://docs.fluentbit.io/manual/data-pipeline/outputs/elasticsearch).

However, the Elasticsearch output plugin fails to recognize this property at runtime.

**To Reproduce**

Configure the pipeline with elastic search output plugin with an `upstream` key :
```yaml
upstream_servers:
- name: my-es-cluster
nodes:
- name: node-01
host: 127.0.0.1
port: 9200

pipeline:
outputs:
- name: es
upstream: my-es-cluster
http_user: myuser
http_passwd: mypassword
```

When starting Fluent Bit, the following error occurs:

```
[error] [config] es: unknown configuration property 'upstream'.
The following properties are allowed: index, type, suppress_type_name, http_user, http_passwd, http_api_key, compress, cloud_id,
cloud_auth, aws_auth, aws_region, aws_sts_endpoint, aws_role_arn, aws_external_id, aws_service_name, aws_profile, logstash_format,
logstash_prefix, logstash_prefix_separator, logstash_prefix_key, logstash_dateformat, time_key, time_key_format, time_key_nanos,
include_tag_key, tag_key, buffer_size, path, pipeline, generate_id, write_operation, id_key, replace_dots, current_time_index, trace_output,
and trace_error.
```

In the list of allowed properties, there is nothing that look like a config for upstreams.

Note: Interestingly, the `host` parameter is also missing from the allowed properties listed in the error message.

**Your Environment**

* Version used: 5.0.3