Randomwalk caves: Reduce 'insure' value from 10 to 2 (#9035)

Randomwalk caves: Reduce 'insure' value from 10 to 2 (#9035)

This value reduces how far randomwalk caves can extend into the
mapchunk padding, to avoid 'out of voxelmanip' cave nodes which cause
flattened cave walls.
Testing shows that a value of 2 (instead of 10) is enough to make
'out of area' nodes extremely rare.
Reducing this value results in a higher chance of overlap and
connection with caves of neighbour mapchunks.