Doc: Clear 8.x series release notes and breaking changes out of main branch

[karenzone]

Related: https://github.com/elastic/docs-projects/issues/257

main = 9.0
8.x series release notes don't belong in the 9.0 branch. Instead we'll populate 9.0 with beta, rc, and 9.0.0 release notes as they become available.

Ideally, we'll remove 8.x release notes sooner rather than later so that we can cut out the wasted effort of forwardporting additional 8.x release notes and prevent inconsistent information experiences across product docs.

PRs to resolve:

• Doc: Reset breaking changes and release notes for 9.0 #16603
• Doc: Roll 6.3 breaking changes under 6.0 series #16706

[karenzone]

Work in progress

• Doc: Reset breaking changes and release notes for 9.0 #16603