Graylog Reports DataNode Not Using Full Heap

The DataNode service is reporting that it's not using the full potential heap that it could erroneously.

## Summary
_Give a brief description of the problem._

### Details
Similar issues were raised here https://github.com/Graylog2/graylog2-server/issues/23250

### Impact
Annoying warning notification that resurfaces every Datanode pod rotation.

## How to Reproduce?

## Environment
All DataNode deployments's can experiences.

### Pre-flight checks
- [ ] `helm lint ./charts/graylog` passes
- [ ] Checked existing issues for duplicates
- [ ] I removed every credential, token, and secret value from the text, logs, and screenshots in this issue

### Steps to reproduce the issue:
1. ...
2. ...

**Expected behavior**
_A clear and concise description of what you expected to happen._

## Notes for maintainers
_Any additional context, logs, or screenshots_

Start with ./charts/graylog and compare its DataNode heap-related configuration with the linked Graylog issue 23250. Run helm lint ./charts/graylog; done should mean the chart no longer causes the recurring full-heap warning during DataNode pod rotations.

Written by the indexing model from the issue text.