Add disruptive change mitigations section to PR template (#4645)

* Add breaking change mitigations section and checklist item

* Remove redundant checklist item

* Broaden language

* Fix copypasta

* Fix heading

.github/PULL_REQUEST_TEMPLATE.md

@@ -36,6 +36,14 @@ List here all the items you have verified BEFORE sending this PR. Please DO NOT
 - [ ] I have added an entry in `./changelog/fragments` using the [changelog tool](https://github.com/elastic/elastic-agent#changelog)
 - [ ] I have added an integration test or an E2E test
 
+## Disruptive User Impact
+
+<!--
+Will the changes introduced by this PR cause disruption to users in any way? If so, please describe what changes users
+could make on their end to nullify or minimize this disruption. Consider impacts in related systems, not just directly
+when using Elastic Agent.
+-->
+
 ## How to test this PR locally
 
 <!-- Recommended