Fix precedence issue causing yielding loop to never yield. (#8922)

There is a bug in `yielding_loop` that causes it to never yield.

## Summary of changes

Fixed the bug. `i + 1 % interval == 0` will always evaluate to `i + 1 ==
0` which is false
([Playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=68e6ca393a02113cb7720115c2842e75)).
This function is called in 2 places
[here](99fa1c3600/pageserver/src/tenant/secondary/scheduler.rs (L389))
and
[here](99fa1c3600/pageserver/src/tenant/secondary/heatmap_uploader.rs (L152))
with `interval == 1000` in both cases.

This may change the performance of the system since now we are yielding
to tokio. Also, this may expose undefined behavior since it is now
possible for tasks to be moved between threads/whatever tokio does to
tasks. However, this was the intention of the author of the code.

libs/utils/src/yielding_loop.rs

@@ -23,7 +23,7 @@ where
     for (i, item) in iter.enumerate() {
         visitor(item);
 
-        if i + 1 % interval == 0 {
+        if (i + 1) % interval == 0 {
             tokio::task::yield_now().await;
             if cancel.is_cancelled() {
                 return Err(YieldingLoopError::Cancelled);