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
PARSER.declareString((response, value) -> {}, new ParseField("tagline"));
        PARSER.declareObject((response, value) -> {
            final String buildType = (String) value.get("build_type");
            response.build = new Build(
                /*
                 * Be lenient when reading on the wire, the enumeration values from other versions might be different than what
                 * we know.
                 */
                buildType == null ? Build.Type.UNKNOWN : Build.Type.fromDisplayName(buildType, false),
                (String) value.get("build_hash"),
                (String) value.get("build_date"),
                (boolean) value.get("build_snapshot"),
                (String) value.get("number")
            );
            response.version = Version.fromString(
                ((String) value.get("number")).replace("-SNAPSHOT", "").replaceFirst("-(alpha\\d+|beta\\d+|rc\\d+)", "")
            );
        }, (parser, context) -> parser.map(), new ParseField("version"));
    }
    public static MainResponse fromXContent(XContentParser parser) {
        return PARSER.apply(parser, null);
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }