9.0.0-beta release notes

[nastasha-solomon]

Description

Generate the 9.0.0-beta release notes and publish them before or on the release day.

To-do

• Draft the release notes
• Add [DO NOT MERGE] 9.0.0-beta release notes #6520 the docs release issue https://github.com/elastic/dev/issues/2995
• Known issues: ❓
• Set up the Security release notes in 9.0 branch in the stack-docs repo: Adds release notes placeholders for Security 9.0.0-beta1 stack-docs#2955
• Add the content in to this file.
• Merge updates to the release-notes-security.asciidoc file once completed

[nastasha-solomon]

Release notes merged.