Missing breaking change notice for docker input in 8.x

[henrikno]

We upgraded beats from 7.17.22 to 8.15.3, and get this error:

Auto discover config check failed for config '{\n \"close_timeout\": \"2h\",\n \"containers\": {\n \"ids\": \"762ef0be6d9fe0f808fc29bd7bcf4eb68b661ee820d21bdb466c62c7f10ff177\",\n \"path\": \"/hostfs/mnt/data/docker/containers\"\n },\n \"ignore_older\": \"6h\",\n \"tags\": [\n \"docker\"\n ],\n \"type\": \"docker\"\n}', won't start runner, err: Error creating input. No such input type exist: 'docker'

So that's how we discovered that this input was removed, even though we read through breaking changes:
https://www.elastic.co/guide/en/beats/libbeat/current/breaking-changes-8.0.html

https://www.elastic.co/guide/en/beats/libbeat/current/release-notes.html
https://www.elastic.co/guide/en/beats/libbeat/current/release-notes-8.0.0.html
I also checked every 8.x.x release notes and there's no mention of it being removed.

So I went investigating:
It was deprecated in 7.2 #12162
Then removed here #28817 for 8.0.
It did add a notice to the changelog, but it must have disappeared along the way.

Combing through git log it got removed here https://github.com/elastic/beats/pull/29391/files#diff-867eb8a32d99eda9bb244b7b86d7abd0031618bae71d0276fe139fe697c3063dL69, and not added anywhere else.

We should add the breaking change to the docs.
Also check if there are other breaking changes that got lost in this PR.