### 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
@Override
        public SourceFieldMapper build() {
            if (enabled.getValue().explicit() && mode.get() != null) {
                throw new MapperParsingException("Cannot set both [mode] and [enabled] parameters");
            }
            if (isDefault()) {
                return indexMode == IndexMode.TIME_SERIES ? TSDB_DEFAULT : DEFAULT;
            }
            SourceFieldMapper sourceFieldMapper = new SourceFieldMapper(
                mode.get(),
                enabled.get(),
                includes.getValue().toArray(String[]::new),
                excludes.getValue().toArray(String[]::new),
                indexMode
            );
            if (indexMode != null) {
                indexMode.validateSourceFieldMapper(sourceFieldMapper);
            }
            return sourceFieldMapper;
        }
        private IndexMode getIndexMode() {
            return indexMode;
        }
    }
    public static final TypeParser PARSER = new ConfigurableTypeParser(
        c -> c.getIndexSettings().getMode() == IndexMode.TIME_SERIES ? TSDB_DEFAULT : DEFAULT,
        c -> new Builder(c.getIndexSettings().getMode())
    );
    static final class SourceFieldType extends MappedFieldType {
        private SourceFieldType(boolean enabled) {