Grafana dashboards: updates to the "Disk space available before publishers blocked" panel

[FSociety07]

Proposed Changes

--- PromQL query change in the Grafana dashboard ---
This change helps visualise the headroom (the gap before blocking) rather than just the raw available space.
The current query just displays the raw available memory. The changed query displays the actual memory that is available before the publishers start getting blocked.
The updated query is now in line with the title of the panel that reads "Disk space available before publishers blocked".

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• [X ] Bug fix (non-breaking change which fixes issue #NNNN)
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