pageserver: increase default max_size_entries limit for basebackup cache (#12343)

## Problem
Some pageservers hit `max_size_entries` limit in staging with only ~25
MiB storage used by basebackup cache. The limit is too strict. It should
be safe to relax it.

- Part of https://github.com/neondatabase/cloud/issues/29353

## Summary of changes
- Increase the default `max_size_entries` from 1000 to 10000

libs/pageserver_api/src/config.rs

@@ -420,7 +420,7 @@ impl Default for BasebackupCacheConfig {
             cleanup_period: Duration::from_secs(60),
             max_total_size_bytes: 1024 * 1024 * 1024, // 1 GiB
             // max_entry_size_bytes: 16 * 1024 * 1024,   // 16 MiB
-            max_size_entries: 1000,
+            max_size_entries: 10000,
             prepare_channel_size: 100,
         }
     }