DataNode: Set heap size automatically

## What?

IMO DataNode should, by default, set the heap size automatically.

## Why?

I somehow completely missed configuring `opensearch_heap` when I converted to Data Node and did not even notice my nodes were running with 1g heap. Thankfully https://github.com/Graylog2/graylog2-server/pull/20168 caught this. But this got me thinking: Why can't or shouldn't this default to 50% ram? There is precedent for this with how we automatically set the buffer processors.

Graylog admins can (and should) still configure `opensearch_heap`, however humans make mistakes and missing things.