[Docs] Remove 9.0 breaking changes from 8.x docs

[florent-leborgne]

This PR removes 9.0 entries from the 8.x breaking changes list

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Pinging @elastic/kibana-docs (Team:Docs)

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[wajihaparvez]

buildkite test this

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cc @wajihaparvez

[lcawl]

The build failure:

INFO:build_docs:  Kibana [8.x]: packages/kbn-doc-links/src/get_doc_links.ts contains broken links to:
--
  | INFO:build_docs:   - en/kibana/8.x/breaking-changes-summary.html#breaking-199656
  | INFO:build_docs:   - en/kibana/8.x/breaking-changes-summary.html#breaking-201004
  | INFO:build_docs:   - en/kibana/8.x/breaking-changes-summary.html#breaking-201550

Is due to the entries in the doc link service like this:

kibana/packages/kbn-doc-links/src/get_doc_links.ts

Line 726 in 7c0c54f

deprecatedV1Endpoints: `${KIBANA_DOCS}breaking-changes-summary.html#breaking-199656`,

It looks like they're used in deprecation messages in the product, so we either need to leave them or else change them to point to "master" and then change again when V9 is forked.

[wajihaparvez]

Based on more offline discussions with @lcawl, to avoid any confusion, we'll revisit this in January when we have a definitive URL for v9.0.0. Leaving this PR open for now.

Thanks, Lisa!

[kapral18]

Please do not merge this pull request. We disabled auto-merge because we are trying to merge a this big PR as part of sustainable architecture migration which is impossible with ever increasing stream of backports. We will resume the automerge after our PR is merged. Reach out to #sustainable-kibana-architecture for more info.

[florent-leborgne]

It looks like they're used in deprecation messages in the product, so we either need to leave them or else change them to point to "master" and then change again when V9 is forked.

Hmm yes I overlooked this. I think we should wait to get the definitive URL in the V9 upgrade docs, then update the links of the 8.x get link service to that new v9 URL. It's ok to leave this file as is for now anyway.

[florent-leborgne]

Closing this, will revisit later