[Doc] remove notes related to 7.x for 9.0

#### Tell us about the issue
Review notes relates to Stack version 7.x and remove them for version 9

**Description:**
Some notes give extra information about old versions which could be too much detail for version 9
Please review those notes and remove them if appropriate

**URL:**

Example: https://www.elastic.co/guide/en/logstash/current/configuring-centralized-pipelines.html

```
Starting with Elasticsearch version 7.10.0, the logstash_admin role inherits the manage_logstash_pipelines cluster privilege for centralized pipeline management. If a user has created their own roles and granted them access to the .logstash index, those roles will continue to work in 7.x but will need to be updated for 8.0
```

**Anything else?**