### Elasticsearch Version

8.7.0

### Installed Plugins

_No response_

### Java Version

_bundled_

### OS Version

whatever

### Problem Description

Enable TSDB and disable source.

### Steps to Reproduce

```
curl -uelastic:password -HContent-Type:application/json -XPUT localhost:9200/test?pretty -d'{
  "settings": {
    "mode": "time_series",
    "routing_path": "foo"
  },
  "mappings": {
    "_source": {
      "enabled": false
    }
  }
}'
```

The error message you get is `Cannot set both [mode] and [enabled] parameters` but we never tried to set `mode`. I think maybe we want `time series indices only support synthetic source` like we give if you try to disable _source in tsdb.

### Logs (if relevant)

_No response_
server/src/main/java/org/elasticsearch/index/mapper/SourceFieldMapper.java
} else {
            return originalSource;
        }
    }
    @Override
    protected String contentType() {
        return CONTENT_TYPE;
    }
    @Override
    public FieldMapper.Builder getMergeBuilder() {
        return new Builder(indexMode).init(this);
    }
    /**
     * Build something to load source {@code _source}.
     */
    public SourceLoader newSourceLoader(Mapping mapping) {
        if (mode == Mode.SYNTHETIC) {
            return new SourceLoader.Synthetic(mapping);
        }
        return SourceLoader.FROM_STORED_SOURCE;
    }
    public boolean isSynthetic() {
        return mode == Mode.SYNTHETIC;
    }
    @Override
    public SourceLoader.SyntheticFieldLoader syntheticFieldLoader() {
        return SourceLoader.SyntheticFieldLoader.NOTHING;
    }
}