chore: EXC-1822: Increase max number of sandboxes to 7k (#3164)

In light of the slight performance degradations we have noticed on some
of the high-load subnets with slow FR which recovers after a few
minutes, the execution team would like to propose to upgrade the
affected subnets to include this change early next week. Although the
issue is certainly not critical, it creates some noise and we believe
that it should be better to allow everyone to have a page-less holiday
period.

The change should be very safe as it basically increases the cache of
sandboxes we keep around which is possible since a few other
improvements that have already been merged a couple weeks ago. It should
allow us to absorb better these cases where we’ve got some extra load by
canisters that have not been executed for a while before we max out the
sandbox cache.

The hope would be to elect a new replica version based on the latest
deployed RC, and then deploy it to the potentially affected subnets
early next week.

Author: berestovskyy

rs/config/src/embedders.rs

@@ -44,7 +44,7 @@ const DEFAULT_PAGE_ALLOCATOR_THREADS: usize = 8;
 
 /// Sandbox process eviction ensures that the number of sandbox processes is
 /// always below this threshold.
-pub(crate) const DEFAULT_MAX_SANDBOX_COUNT: usize = 5_000;
+pub(crate) const DEFAULT_MAX_SANDBOX_COUNT: usize = 7_000;
 
 /// A sandbox process may be evicted after it has been idle for this
 /// duration and sandbox process eviction is activated.