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

[kaisecheng]

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?