pageserver: decrease MAX_SHARDS in utilization (#12668)

## Problem

When tenants have a lot of timelines, the number of tenants that a
pageserver can comfortably handle goes down. Branching is much more
widely used in practice now than it was when this code was written, and
we generally run pageservers with a few thousand tenants (where each
tenant has many timelines), rather than the 10k-20k we might have done
historically.

This should really be something configurable, or a more direct proxy for
resource utilization (such as non-archived timeline count), but this
change should be a low effort improvement.

## Summary of changes

* Change the target shard count (MAX_SHARDS) to 2500 from 5000 when
calculating pageserver utilization (i.e. a 200% overcommit now
corresponds to 5000 shards, not 10000 shards)

Co-authored-by: John Spray <john.spray@databricks.com>

pageserver/src/utilization.rs

@@ -52,7 +52,7 @@ pub(crate) fn regenerate(
     };
 
     // Express a static value for how many shards we may schedule on one node
-    const MAX_SHARDS: u32 = 5000;
+    const MAX_SHARDS: u32 = 2500;
 
     let mut doc = PageserverUtilization {
         disk_usage_bytes: used,