Issues with very large bodies don't get ingested into issues-elastic-kibana in github-stats

Here's an example of an issue with huge body which isn't currently in the `issues-elastic-kibana` index;

https://github.com/elastic/kibana/issues/105356

I'm certainly fine with the body getting truncated to some reasonable size. I suppose this index is used by a lot of people in both the Kibana and Elasticsearch teams, so maybe truncated to the largest size that works is best?