Doc: Update landing page for Breaking changes

[karenzone]

The landing page for Breaking Changes should be updated to:

• Remove old content and links
• Clean up content and refine wording to be relevant for 8.0 release, removing older versions, etc.

Link: https://www.elastic.co/guide/en/logstash/current/breaking-changes.html

[karenzone]

@alexander-marquardt I started working on this issue in preparation for the 8.0 release, and then ran into some decisions I'd like to get your input on. Let's sync on this, please.

[karenzone]

Closed with merge of #13693