fix(storage-scrubber): valid layermap error degrades to warning (#9902)

Valid layer assumption is a necessary condition for a layer map to be
valid. It's a stronger check imposed by gc-compaction than the actual
valid layermap definition. Actually, the system can work as long as
there are no overlapping layer maps. Therefore, we degrade that into a
warning.

Signed-off-by: Alex Chi Z <chi@neon.tech>

storage_scrubber/src/checks.rs

@@ -128,7 +128,7 @@ pub(crate) async fn branch_cleanup_and_check_errors(
 
                     let layer_names = index_part.layer_metadata.keys().cloned().collect_vec();
                     if let Some(err) = check_valid_layermap(&layer_names) {
-                        result.errors.push(format!(
+                        result.warnings.push(format!(
                             "index_part.json contains invalid layer map structure: {err}"
                         ));
                     }