pageserver: quieten "Failed to get info about AUX files" (#5669)

## Problem

This line caused lots of errors to be emitted for healthy tenants.

## Summary of changes

Downgrade to debug, since it is an expected code path we'll take for
tenants at startup.

pageserver/src/pgdatadir_mapping.rs

@@ -1202,7 +1202,8 @@ impl<'a> DatadirModification<'a> {
         let mut dir = match self.get(AUX_FILES_KEY, ctx).await {
             Ok(buf) => AuxFilesDirectory::des(&buf)?,
             Err(e) => {
-                warn!("Failed to get info about AUX files: {}", e);
+                // This is expected: historical databases do not have the key.
+                debug!("Failed to get info about AUX files: {}", e);
                 AuxFilesDirectory {
                     files: HashMap::new(),
                 }