Don't require brief for deprecated attributes, auto-generate it in markdown and code

We currently do this: 

```yaml
brief: 'Deprecated, use `server.address`, `server.port` attributes instead.'
deprecated: "Replaced by `server.address` and `server.port`."
```

It'd be nice to avoid duplication - https://github.com/open-telemetry/semantic-conventions/pull/1418#discussion_r1766981411