### 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_
x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/cluster/metadata/MetadataMigrateToDataTiersRoutingService.java
Template currentInnerTemplate = composableTemplate.template();
                    ComposableIndexTemplate.Builder migratedComposableTemplateBuilder = new ComposableIndexTemplate.Builder();
                    Settings.Builder settingsBuilder = Settings.builder().put(settings);
                    settingsBuilder.remove(requireRoutingSetting);
                    settingsBuilder.remove(includeRoutingSetting);
                    settingsBuilder.remove(excludeRoutingSetting);
                    Template migratedInnerTemplate = new Template(
                        settingsBuilder.build(),
                        currentInnerTemplate.mappings(),
                        currentInnerTemplate.aliases(),
                        currentInnerTemplate.lifecycle()
                    );
                    migratedComposableTemplateBuilder.indexPatterns(composableTemplate.indexPatterns());
                    migratedComposableTemplateBuilder.template(migratedInnerTemplate);
                    migratedComposableTemplateBuilder.componentTemplates(composableTemplate.composedOf());
                    migratedComposableTemplateBuilder.priority(composableTemplate.priority());
                    migratedComposableTemplateBuilder.version(composableTemplate.version());
                    migratedComposableTemplateBuilder.metadata(composableTemplate.metadata());
                    migratedComposableTemplateBuilder.dataStreamTemplate(composableTemplate.getDataStreamTemplate());
                    migratedComposableTemplateBuilder.allowAutoCreate(composableTemplate.getAllowAutoCreate());
                    migratedComposableTemplateBuilder.ignoreMissingComponentTemplates(
                        composableTemplate.getIgnoreMissingComponentTemplates()
                    );
                    mb.put(templateEntry.getKey(), migratedComposableTemplateBuilder.build());
                    migratedComposableTemplates.add(templateEntry.getKey());
                }
            }
        }
        return migratedComposableTemplates;
    }