### 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/test/java/org/elasticsearch/xpack/cluster/metadata/MetadataMigrateToDataTiersRoutingServiceTests.java
.metadata(
                Metadata.builder()
                    .put(legacyTemplateWithRequireRouting)
                    .put("composable-template-with-require-routing", composableTemplateWithRequireRouting)
                    .put("component-with-require-and-include-routing", compTemplateWithRequireAndIncludeRoutings)
                    .put("component-template-without-custom-routing", compTemplateWithoutCustomRoutings)
                    .build()
            )
            .build();
        Metadata.Builder mb = Metadata.builder(clusterState.metadata());
        MetadataMigrateToDataTiersRoutingService.MigratedTemplates migratedTemplates = MetadataMigrateToDataTiersRoutingService
            .migrateIndexAndComponentTemplates(mb, clusterState, nodeAttrName);
        assertThat(migratedTemplates.migratedLegacyTemplates, is(List.of("template-with-require-routing")));
        assertThat(migratedTemplates.migratedComposableTemplates, is(List.of("composable-template-with-require-routing")));
        assertThat(migratedTemplates.migratedComponentTemplates, is(List.of("component-with-require-and-include-routing")));
    }
    private String getWarmPhaseDef() {
        return Strings.format("""
            {
              "policy": "%s",
              "phase_definition": {
                "min_age": "0m",
                "actions": {
                  "allocate": {
                    "number_of_replicas": "0",
                    "require": {
                      "data": "cold"
                    }
                  },
                  "set_priority": {