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
}
    public MainResponse(String nodeName, Version version, ClusterName clusterName, String clusterUuid, Build build) {
        this.nodeName = nodeName;
        this.version = version;
        this.clusterName = clusterName;
        this.clusterUuid = clusterUuid;
        this.build = build;
    }
    public String getNodeName() {
        return nodeName;
    }
    public Version getVersion() {
        return version;
    }
    public ClusterName getClusterName() {
        return clusterName;
    }
    public String getClusterUuid() {
        return clusterUuid;
    }
    public Build getBuild() {
        return build;
    }
    @Override
    public void writeTo(StreamOutput out) throws IOException {
        out.writeString(nodeName);
        Version.writeVersion(version, out);
        clusterName.writeTo(out);
        out.writeString(clusterUuid);
        Build.writeBuild(build, out);