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
@Override
    protected MainResponse createTestInstance() {
        String clusterUuid = randomAlphaOfLength(10);
        ClusterName clusterName = new ClusterName(randomAlphaOfLength(10));
        String nodeName = randomAlphaOfLength(10);
        final String date = new Date(randomNonNegativeLong()).toString();
        Version version = VersionUtils.randomIndexCompatibleVersion(random());
        Build build = new Build(Build.Type.UNKNOWN, randomAlphaOfLength(8), date, randomBoolean(), version.toString());
        return new MainResponse(nodeName, version, clusterName, clusterUuid, build);
    }
    @Override
    protected Writeable.Reader<MainResponse> instanceReader() {
        return MainResponse::new;
    }
    @Override
    protected MainResponse doParseInstance(XContentParser parser) {
        return MainResponse.fromXContent(parser);
    }
    public void testToXContent() throws IOException {
        String clusterUUID = randomAlphaOfLengthBetween(10, 20);