A tag is required to reuse Elasticsearch breaking changes in the Stack
Guide. To display properly, the breaking changes must use external
links rather than xrefs.
This PR correctly places those tags for reuse. It also replaces
several xrefs with external links for reuse.
Adds `//tag::notable-breaking-changes[]` to several breaking changes
files. This tag is required for reuse in the [Elastic Stack Installation
and Upgrade Guide][0].
Also replaces a relative xref with an external links. External links are
required for reuse in the Installation and Upgrade Guide.
[0]: https://www.elastic.co/guide/en/elastic-stack/master/index.html