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
private String nodeName;
    private Version version;
    private ClusterName clusterName;
    private String clusterUuid;
    private Build build;
    MainResponse() {}
    MainResponse(StreamInput in) throws IOException {
        super(in);
        nodeName = in.readString();
        version = Version.readVersion(in);
        clusterName = new ClusterName(in);
        clusterUuid = in.readString();
        build = Build.readBuild(in);