fix: use selectorLabels

[monrax]

Summary

• Fix: Add selector labels to both Service and PBD resources in order to prevent collisions with existing releases.
• Refactor: Improve readability while keeping the same logic in PDB conditional.

What changed

• Add selectorLabels to service/graylog.yaml and service/datanode.yaml
• Add selectorLabels to pdb/graylog.yaml and pdb/datanode.yaml

Linked issues

Closes #71
Closes #73

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

• Linter check passes: helm lint ./charts/graylog
• Helm renders local template sucessfully: helm template graylog ./charts/graylog --validate

Installation

• Fresh installation completes successfully: helm install graylog ./charts/graylog
• All pods reach Running state: kubectl rollout status statefulset/graylog
• Helm tests pass: helm test graylog

Functional (if applicable)

• Web UI accessible and login works
• DataNodes visible in System > Cluster Configuration
• 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

• Follow all steps in "How to reproduce?" section in linked issues, and verify no errors are encountered.

Notes for reviewers

• Verify all tests above pass
• Sync up with the author before merging
• The commit history should be preserved - use rebase-merge or standard merge options when applicable