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
final Build current = Build.CURRENT;
        Build build = new Build(current.type(), current.hash(), current.date(), current.isSnapshot(), current.qualifiedVersion());
        Version version = Version.CURRENT;
        MainResponse response = new MainResponse("nodeName", version, new ClusterName("clusterName"), clusterUUID, build);
        XContentBuilder builder = XContentFactory.jsonBuilder();
        response.toXContent(builder, ToXContent.EMPTY_PARAMS);
        assertEquals(
            XContentHelper.stripWhitespace(
                """
                    {
                        "name": "nodeName",
                        "cluster_name": "clusterName",
                        "cluster_uuid": "%s",
                        "version": {
                            "number": "%s",
                            "build_type": "%s",
                            "build_hash": "%s",
                            "build_date": "%s",
                            "build_snapshot": %s,
                            "lucene_version": "%s",
                            "minimum_wire_compatibility_version": "%s",
                            "minimum_index_compatibility_version": "%s"
                        },
                        "tagline": "You Know, for Search"
                    }
                    """.formatted(
                    clusterUUID,
                    build.qualifiedVersion(),
                    current.type().displayName(),
                    current.hash(),