[DOCS] Add 7.1, 7.2, 7.3, and 8.0 breaking changes files

[dedemorton]

Adds 8.0 and 7.3 breaking changes. The files will not be hooked up to the navigation until we have an 8.x and 7.3 branches.

Also forward ports changes added in #12326 and #12322.

This PR must be merged at the same time as elastic/stack-docs#350

[lcawl]

Would it save time to create a 7.3.0 page for the 7.x branch now too?

[lcawl]

FYI in other products' docs we don't include the 7.* release notes or highlights or breaking changes in the 8.* books. e.g. https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking-changes.html
Just mentioning in case you want to omit the 7.* pages. If that's not feasible because of the merge strategy in this repo, that's fine.

[lcawl]

The Beats Platform Reference builds successfully, but I added two comments for consideration.

[dedemorton]

Just mentioning in case you want to omit the 7.* pages. If that's not feasible because of the merge strategy in this repo, that's fine.

Yes, that's exactly why I'm including the 7.x pages. I don't want to mess up the automerge from master > 7.x.

[dedemorton]

@lcawl I've added the 7.3 file, too. Once we create an 8.x branch, we can clean out the 7.x files, but for now, let's keep all the 7.x files in master so the merge is simpler.