src/mon/MgrStatMonitor: fix invalid iterator increment in calc_pool_availability()

src/mon/MgrStatMonitor: fix invalid iterator increment in calc_pool_availability()

Erasing entries from `pool_availability` inside a range-for
loop invalidated the hidden iterator, triggering an
“Invalid read” under Valgrind.

- Use `std::erase_if(pool_availability, predicate)` for
  atomic removal.
- Refactor the stats-update loop to use structured bindings
  and a clear `++it` for readability.

Fixes: https://tracker.ceph.com/issues/71271
Signed-off-by: Nitzan Mordechai <nmordec@redhat.com>