If a node exceeds the flood-stage disk watermark then we add a block to all of
its indices to prevent further writes as a last-ditch attempt to prevent the
node completely exhausting its disk space. However today this block remains in
place until manually removed, and this block is a source of confusion for users
who current have ample disk space and did not even realise they nearly ran out
at some point in the past.
This commit changes our behaviour to automatically remove this block when a
node drops below the high watermark again. The expectation is that the high
watermark is some distance below the flood-stage watermark and therefore the
disk space problem is truly resolved.
Fixes#39334
cd304c4def