Add relates to to our pr template for multi-pr issues (#1406)

## Description

Update our pr template to include a relates to issue #.

## Related Issue

Fixes #n/a

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [x] Other (security config, docs update, etc)

## Checklist before merging

- [x] Test, docs, adr added or updated as needed
- [x] [Contributor Guide
Steps](https://github.com/defenseunicorns/zarf/blob/main/CONTRIBUTING.md#developer-workflow)
followed

.github/pull_request_template.md

@@ -5,6 +5,8 @@
 ## Related Issue
 
 Fixes #
+<!-- or -->
+Relates to #
 
 ## Type of change