In https://github.com/elastic/elasticsearch/pull/85796 the `version.build_flavor` field was removed from the `/` API response. This API is marked as `stable` in the [API spec stub](https://github.com/elastic/elasticsearch/blob/master/rest-api-spec/src/main/resources/rest-api-spec/api/info.json) and thus this removal should be considered a regression. The removal has started [causing issues for users of clients](https://github.com/elastic/elasticsearch-java/issues/335) when they connect to 8.3.0+ of Elasticsearch.

My proposed fix is to add back the field with a static value of `default` and to change the removal of the field to the next major version instead.
server/src/main/java/org/elasticsearch/action/main/MainResponse.java
}
    @Override
    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
        builder.startObject();
        builder.field("name", nodeName);
        builder.field("cluster_name", clusterName.value());
        builder.field("cluster_uuid", clusterUuid);
        builder.startObject("version")
            .field("number", build.qualifiedVersion())
            .field("build_type", build.type().displayName())
            .field("build_hash", build.hash())
            .field("build_date", build.date())
            .field("build_snapshot", build.isSnapshot())
            .field("lucene_version", version.luceneVersion.toString())
            .field("minimum_wire_compatibility_version", version.minimumCompatibilityVersion().toString())
            .field("minimum_index_compatibility_version", version.minimumIndexCompatibilityVersion().toString())
            .endObject();
        builder.field("tagline", "You Know, for Search");
        builder.endObject();
        return builder;
    }
    private static final ObjectParser<MainResponse, Void> PARSER = new ObjectParser<>(
        MainResponse.class.getName(),
        true,
        MainResponse::new
    );
    static {
        PARSER.declareString((response, value) -> response.nodeName = value, new ParseField("name"));
        PARSER.declareString((response, value) -> response.clusterName = new ClusterName(value), new ParseField("cluster_name"));
        PARSER.declareString((response, value) -> response.clusterUuid = value, new ParseField("cluster_uuid"));