chore: add note to bug report template (#30934)

### Issue # (if applicable)

N/A

### Reason for this change

Remind users to submit security-related issues using the existing mechanism in GitHub.

### Description of changes

Add note to the top of the template

### Description of how you validated changes

Previewed the template in GitHub (can't find a way to actually view the updated template locally)

### Checklist
- [n/a] 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)

----

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

.github/ISSUE_TEMPLATE/bug-report.yml

@@ -5,6 +5,14 @@ title: "(module name): (short issue description)"
 labels: [bug, needs-triage]
 assignees: []
 body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to fill out this bug report!
+
+        **⚠️ Please read this before filling out the form below:**  
+        If the bug you are reporting is a security-related issue or a security vulnerability,
+        please report it via [Report a security vulnerability](https://github.com/aws/aws-cdk/security/advisories/new) instead of this template.
   - type: textarea
     id: description
     attributes: