pageserver: tweak logs for index_part loading (#6005)

## Problem

On pageservers upgraded to enable generations, these INFO level logs
were rather frequent. If a tenant timeline hasn't written new layers
since the upgrade, it will emit the "No index_part.json*" log every time
it starts.

## Summary of changes

- Downgrade two log lines from info to debug
- Add a tiny unit test that I wrote for sanity-checking that there
wasn't something wrong with our Generation-comparing logic when loading
index parts.

libs/utils/src/generation.rs

@@ -152,3 +152,16 @@ impl Debug for Generation {
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn generation_gt() {
+        // Important that a None generation compares less than a valid one, during upgrades from
+        // pre-generation systems.
+        assert!(Generation::none() < Generation::new(0));
+        assert!(Generation::none() < Generation::new(1));
+    }
+}