### Elasticsearch Version

7.17, 8.x

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

Darwin

### Problem Description

The migrate to data tiers API can sometimes configure an illegal value for the `_tier_preference` setting for partially mounted indices.
This can happen if two conditions are met: 
1. if the partially mounted index doesn't have a `_tier_preference` configured
2. if the migrate to data tiers API doesn't perform any migration for this index (e.g. say this partially mounted index doesn't use the `data` custom attribute name for any allocation filtering, but either uses some other node attribute name or none at all)

In this case this piece of code kicks in to configure a default tier preference https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/cluster/metadata/MetadataMigrateToDataTiersRoutingService.java#L534

This is wrong, and we should configure the `MountSearchableSnapshotRequest.Storage.defaultDataTiersPreference` as the tier preference.

Relates to https://github.com/elastic/elasticsearch/issues/84758

### Steps to Reproduce

1. Mount an index - `partial-test`
2. Remove the `_tier_preference` using
```
PUT partial-test/_settings
{
  "index.routing.allocation.include._tier_preference": null
}
```
3. Call the migrate to data tiers routing API 
```
POST /_ilm/migrate_to_data_tiers
```

### Logs (if relevant)

_No response_
server/src/main/java/org/elasticsearch/cluster/routing/allocation/DataTier.java
}
            }
        }
        @Override
        public void validate(String value, Map<Setting<?>, Object> settings, boolean exists) {
            if (exists && value != null) {
                if (SearchableSnapshotsSettings.isPartialSearchableSnapshotIndex(settings)) {
                    if (value.equals(DATA_FROZEN) == false) {
                        throw new IllegalArgumentException(
                            "only the ["
                                + DATA_FROZEN
                                + "] tier preference may be used for partial searchable snapshots (got: ["
                                + value
                                + "])"
                        );
                    }
                } else {
                    if (value.contains(DATA_FROZEN)) {
                        throw new IllegalArgumentException("[" + DATA_FROZEN + "] tier can only be used for partial searchable snapshots");
                    }
                }
            }
        }
        @Override
        public Iterator<Setting<?>> settings() {
            return dependencies.iterator();
        }
    }
}