chore: add breaking changes to end of PR template (#28941)

### Reason for this change

We require breaking changes to be listed at the end of the PR description (see [here](https://github.com/aws/aws-cdk/blob/b32f47cfd65a5288e7bc2cc5a8a1f8e5f59032f8/CONTRIBUTING.md?plain=1#L620)), adding it to the template makes it easier for contributors to adhere to this 

### Description of changes

Add breaking changes format to end of PR template

### Description of how you validated changes

n/a

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: gracelu0

.github/PULL_REQUEST_TEMPLATE.md

@@ -17,6 +17,14 @@ Closes #<issue number here>.
 ### Checklist
 - [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)
 
+<!--Does this PR includes breaking changes? If it does, list them here in the following format (notice how multiple breaking changes should be formatted):
+
+  ```
+  BREAKING CHANGE: Description of the breaking change, the previous behaviour, and the new behaviour. 
+  * **module-name:** Another breaking change
+  * **module-name:** Yet another breaking change
+  ```
+-->
 ----
 
 *By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*