Add test for QueryExplanation serialization backwards compatibility

[pcsanwald]

See #30390, we should test backwards compatibility for QueryExplanation Serialization.

[elasticmachine]

Pinging @elastic/es-search-aggs

[edemsegbedzi]

I would like to work on this, can I commence?

[pcsanwald]

@Segbedzi sure thing! I think you'll need to sign the contributor agreement, and here are instructions for getting started contributing. I'd be happy to help if you need any tips on this one. Thank you!

[inavuk]

Hi, just wondering if this is being worked on - if so, do you have any other issues for a newbie you could use help on?

[cbuescher]

@inavuk seems like @Segbedzi showed some interest, so you should coordinate with him to avoid double work. Other than that, look at the two documents linked above and check the repo for issues labeled "adoptme" and/or "low hanging fruit" to get started. Also there are several issues labeled "doc" which sometimes are not too hard to get started with and don't involve deep dives into the code, but they can get you familier with our process of handling PRs.
Hope you find something of interest to you.