Enhance PR template with linked issues and testing notes

[monrax]

Summary

Updated the pull request template to include a section for linked issues and a reference to the testing documentation.

What changed

• Minor changes to .github/PULL_REQUEST_TEMPLATE.md

Checklist

• Tests added/updated
• Documentation updated
• This PR includes a new feature
• This PR includes a bugfix
• This PR includes a refactor

Testing Checklist

Static Validation

• helm lint ./charts/graylog passes
• helm template graylog ./charts/graylog --validate passes

Installation

• Fresh installation completes successfully
• All pods reach Running state
• helm test graylog -n graylog passes

Functional (if applicable)

• Web UI accessible and login works
• DataNodes visible in System > Data Nodes
• Inputs can be created and receive data

Upgrade (if applicable)

• Upgrade from previous release succeeds
• Scaling up/down works correctly
• Configuration changes apply correctly

Specific to this PR

• describe what was specifically tested

Notes for reviewers

• Verify all applicable tests above pass
• Sync up with the author before merging