[DOCS] Add Elastic Security breaking changes to Installation and Upgrade Guide for 8.1

[nastasha-solomon]

Description

Add Elastic Security breaking changes to this page in the Elastic Installation and Upgrade Guide:
https://www.elastic.co/guide/en/elastic-stack/8.1/elastic-stack-breaking-changes.html

Our breaking changes are in the 8.1 Release Notes:
https://www.elastic.co/guide/en/security/current/release-notes-header-8.1.0.html#breaking-changes-8.1.0

Notes

• The breaking changes page in the Install & Upgrade Guide is built using includes of tagged regions, so we'll also need to add tags to the relevant region in the Release Notes.