Add ES 8.x support

[ryansch]

In addition to pinning v8 of elasticsearch-ruby, there were two other things I had to consider.

ES 8 does not allow sending a document type. I followed previous work in this repo and used Hash#compact to remove the type param.

elasticsearch-ruby v8 uses the new elastic-transport gem instead of the older elasticsearch-transport one. I added a (hopefully) backwards compatible solution via ELASTIC_TRANSPORT_CLASS.

[ryansch]

The test suite needs to run with ES 8 on this branch but I'm not familiar enough with it to make the right change.

[RouL]

Without having a deeper look into it I'd say replace all occurences of stack-version: 7.x-SNAPSHOT with stack-version: 8.4.0-SNAPSHOT in .github/workflows/*.yml At least that's what is in the elasticsearch-ruby main-branch.