### 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
private static final Logger logger = LogManager.getLogger(DefaultHotAllocationSettingProvider.class);
        @Override
        public Settings getAdditionalIndexSettings(
            String indexName,
            String dataStreamName,
            boolean timeSeries,
            Metadata metadata,
            Instant resolvedAt,
            Settings allSettings,
            List<CompressedXContent> combinedTemplateMappings
        ) {
            Set<String> settings = allSettings.keySet();
            if (settings.contains(TIER_PREFERENCE)) {
                
                return NULL_TIER_PREFERENCE_SETTINGS;
            } else if (settings.stream().anyMatch(s -> s.startsWith(IndexMetadata.INDEX_ROUTING_REQUIRE_GROUP_PREFIX + "."))
                || settings.stream().anyMatch(s -> s.startsWith(IndexMetadata.INDEX_ROUTING_EXCLUDE_GROUP_PREFIX + "."))
                || settings.stream().anyMatch(s -> s.startsWith(IndexMetadata.INDEX_ROUTING_INCLUDE_GROUP_PREFIX + "."))) {
                    
                    logger.debug("index [{}] specifies custom index level routing filtering, skipping tier allocation", indexName);
                    return Settings.EMPTY;
                } else {