Required updates to support es 7.7

[spiritinlife]

I am not sure about this part.

@Override
        public boolean isResultDeterministic() {
            // PositionRecipScript does not use deterministic APIs so that means that results
            // are not cachable? @todo
            return true;
        }

Related resources about this
elastic/elasticsearch#49466
https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-scripting-engine.html

[anti-social]

As I understand it is deterministic as it doesn't use any API like random.

[spiritinlife]

Y that was my understanding also. Lets remove the @todo comment then?

[anti-social]

You can try releases for Elasticsearch 7.7.x:

• https://github.com/anti-social/elasticsearch-rescore-grouping-mixup/releases/tag/v0.2.5-es7.7.0
• https://github.com/anti-social/elasticsearch-rescore-grouping-mixup/releases/tag/v0.2.5-es7.7.1

Please let me know if all is ok with them.