[Upgrade Assistant Meta] Breaking changes issue template (elastic#59745)

* First draft of issue template

* [skip ci] Slight update to issue template

* Add label

.github/ISSUE_TEMPLATE/v8_breaking_change.md

@@ -0,0 +1,36 @@
+---
+name: 8.0 Breaking change
+about: Breaking changes from 7.x -> 8.0
+title: "[Breaking change]"
+labels: Team:Elasticsearch UI, Feature:Upgrade Assistant
+assignees: ''
+
+---
+
+## Change description
+
+**Which release will ship the breaking change?**
+
+<!-- e.g., v7.6.2 -->
+
+**Describe the change. How will it manifest to users?**
+
+**What percentage of users will be affected?**
+
+<!-- e.g., Roughly 75% will need to make changes to x. -->
+
+**What can users to do to address the change manually?**
+
+<!-- If applicable, describe the manual workaround -->
+
+**How could we make migration easier with the Upgrade Assistant?**
+
+**Are there any edge cases?**
+
+## Test Data
+
+Provide test data. We can’t build a solution without data to test it against.
+
+## Cross links
+
+Cross-link to relevant [Elasticsearch breaking changes](https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking-changes-8.0.html).