chore(pageserver): warn on delete non-existing file (#7847)

Consider the following sequence of migration:

```
1. user starts compute
2. force migrate to v2
3. user continues to write data
```

At the time of (3), the compute node is not aware that the page server
does not contain replication states any more, and might continue to
ingest neon-file records into the safekeeper. This will leave the
pageserver store a partial replication state and cause some errors. For
example, the compute could issue a deletion of some aux files in v1, but
this file does not exist in v2. Therefore, we should ignore all these
errors until everyone is migrated to v2.

Also note that if we see this warning in prod, it is likely because we
did not fully suspend users' compute when flipping the v1/v2 flag.

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

pageserver/src/pgdatadir_mapping.rs

@@ -1552,7 +1552,7 @@ impl<'a> DatadirModification<'a> {
                     self.tline.aux_file_size_estimator.on_add(content.len());
                     new_files.push((path, content));
                 }
-                (None, true) => anyhow::bail!("removing non-existing aux file: {}", path),
+                (None, true) => warn!("removing non-existing aux file: {}", path),
             }
             let new_val = aux_file::encode_file_value(&new_files)?;
             self.put(key, Value::Image(new_val.into()));