### 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
false,
            m -> Arrays.asList(toType(m).includes)
        );
        private final Parameter<List<String>> excludes = Parameter.stringArrayParam(
            "excludes",
            false,
            m -> Arrays.asList(toType(m).excludes)
        );
        private final IndexMode indexMode;
        public Builder(IndexMode indexMode) {
            super(Defaults.NAME);
            this.indexMode = indexMode;
        }
        public Builder setSynthetic() {
            this.mode.setValue(Mode.SYNTHETIC);
            return this;
        }
        @Override
        protected Parameter<?>[] getParameters() {
            return new Parameter<?>[] { enabled, mode, includes, excludes };
        }
        private boolean isDefault() {
            if (mode.get() != null) {
                return false;
            }
            if (enabled.get().value() == false) {
                return false;
            }
            return includes.getValue().isEmpty() && excludes.getValue().isEmpty();
        }