Support HAProxy 2.x

**Describe the enhancement:**

According to https://www.elastic.co/guide/en/beats/metricbeat/current/metricbeat-module-haproxy.html#_compatibility_22, the HAProxy module supports HAProxy versions 1.6, 1.7 and 1.8.

Running HAProxy 2.6, the following error occurs when Metricbeat tries to index events:

```
WARN [elasticsearch] elasticsearch/client.go:414 Cannot index event publisher.Event{Content:beat.Event{Timestamp:time.Date(2023, time.March, 17, 21, 36, 50, 235561684, time.Local), Meta:null, Fields: [...] (status=400): {"type":"mapper_parsing_exception","reason":"failed to parse field [haproxy.stat.check.agent.last] of type [long] in document with id 'LbZK8YYBniYVjaQFG-SZ'. Preview of field's value: 'via agent : up'","caused_by":{"type":"illegal_argument_exception","reason":"For input string: \"via agent : up\""}}, dropping event!
```

**Describe a specific use case for the enhancement or feature:**

All 1.x versions are EOL.