Docs bug - os_service is not implemented on linux.

#### Overview of the Issue

The documentation clearly states that os_service supports both windows and systemd service types. However systemd support is unimplemented. Either the docs should be updated to reflect reality, or the support should be implemented.

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#### Reproduction Steps

### Consul info for both Client and Server

N/A

### Operating system and Environment details

Debian 11, amd64.

### Log Fragments

```
Oct 02 18:12:15 consul-demo1 consul[14100]: 2023-10-02T18:12:15.469Z [ERROR] agent: error creating OS Service client: error="not implemented"
```

Start by locating the os_service implementation and the documentation that states support for Windows and systemd service types. Confirm whether systemd support is intended before choosing between correcting the documentation and implementing the missing behavior; done means the documented behavior matches Consul and the reported not-implemented error is resolved.

Written by the indexing model from the issue text.