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/test/java/org/elasticsearch/action/main/MainResponseTests.java
current.date(),
                    current.isSnapshot(),
                    version.luceneVersion.toString(),
                    version.minimumCompatibilityVersion().toString(),
                    version.minimumIndexCompatibilityVersion().toString()
                )
            ),
            Strings.toString(builder)
        );
    }
    @Override
    protected MainResponse mutateInstance(MainResponse mutateInstance) {
        String clusterUuid = mutateInstance.getClusterUuid();
        Build build = mutateInstance.getBuild();
        Version version = mutateInstance.getVersion();
        String nodeName = mutateInstance.getNodeName();
        ClusterName clusterName = mutateInstance.getClusterName();
        switch (randomIntBetween(0, 4)) {
            case 0 -> clusterUuid = clusterUuid + randomAlphaOfLength(5);
            case 1 -> nodeName = nodeName + randomAlphaOfLength(5);
            case 2 ->
                
                build = new Build(Build.Type.UNKNOWN, build.hash(), build.date(), build.isSnapshot() == false, build.qualifiedVersion());
            case 3 -> version = randomValueOtherThan(version, () -> VersionUtils.randomVersion(random()));
            case 4 -> clusterName = new ClusterName(clusterName + randomAlphaOfLength(5));
        }
        return new MainResponse(nodeName, version, clusterName, clusterUuid, build);
    }
}