fix: Raise `start-druid` middleManager memory to 256m (#19582)

* Raise start-druid middleManager memory floor to 256m

The auto memory split in start-druid gives the middleManager the lowest
weight, so on a moderately sized machine it can be awarded a very small
heap: at `-m 16g` it gets only ~68m. The middleManager does not just fork
peons, it also runs an embedded HTTP server that proxies task reports and
logs back to callers (for example the web console polling a running query).
Under that proxying load a ~68m heap is exhausted, the middleManager exits
with an OutOfMemoryError, and the task it owns is reported as having
"disappeared on the worker".

Raise the middleManager minimum from 64m to 256m. 256m matches the heap
already shipped for the medium/large/xlarge single-server example configs.
The floor only affects deployments small enough that the proportional split
would fall below it (roughly under 60g total); larger deployments are
unchanged.

* Drop the explanatory comment on the middleManager floor