chore: update defaults for timeline_detach_ancestor (#7779)

by having 100 copy operations in flight twe climb up to 2500 requests
per min or 41/s. This is still probably less than is allowed, but fast
enough for our purposes.

pageserver/src/tenant/timeline/detach_ancestor.rs

@@ -56,7 +56,7 @@ impl Default for Options {
     fn default() -> Self {
         Self {
             rewrite_concurrency: std::num::NonZeroUsize::new(2).unwrap(),
-            copy_concurrency: std::num::NonZeroUsize::new(10).unwrap(),
+            copy_concurrency: std::num::NonZeroUsize::new(100).unwrap(),
         }
     }
 }