### 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
public static final String NAME = SourceFieldMapper.NAME;
        public static final FieldType FIELD_TYPE = new FieldType();
        static {
            FIELD_TYPE.setIndexOptions(IndexOptions.NONE); 
            FIELD_TYPE.setStored(true);
            FIELD_TYPE.setOmitNorms(true);
            FIELD_TYPE.freeze();
        }
    }
    private static SourceFieldMapper toType(FieldMapper in) {
        return (SourceFieldMapper) in;
    }
    public static class Builder extends MetadataFieldMapper.Builder {