Configuring source mode in mappings is deprecated and replaced by an index setting

Index source mode was previously configured in mappings as follows:

"mappings": {
    "source": {
        "mode": "synthetic"
    },
    "foo": {
        "type": "keyword"
    },
    "bar": {
        "type": "keyword"
    }
}

Starting with version 8.18, this method for configuring the source mode is considered deprecated and replaced by index setting index.mapping.source.mode that accepts the same values: stored, synthetic and disabled. The index setting can be used as follows:

"settings": {
    "index.mapping.source.mode": "synthetic"
}
"mappings": {
    "foo": {
        "type": "keyword"
    },
    "bar": {
        "type": "keyword"
    }
}

Existing indexes using the deprecated method for configuring the source mode are not affected, but creating new indexes raises warnings and will not be supported in a future release. This can be an issue for data streams and other indexes that get regularly generated using component templates. To avoid these problems, identify all affected component templates, as shown in the deprecation info API, and update them to use setting index.mapping.source.mode instead of mappings.source.mode, according to the examples above.