Replaces Elasticsearch::Transport with Elastic::Transport

Removes type
Updates option hash/keyword argument expectations

Updates usage of client

...whitespace cleanup

I need to understand if we still need MultiModel or if it was based on
the document_type which is no longer a thing in Elasticsearch.

- Adds Ruby 3.0 tests
- Removes Ruby 2.6 and 2.7 tests

The goal of ClassMethodsProxy is to avoid polluting the target's namespace,
but it was possible to do this by accident when calling `class_eval` before
ActiveSupport was completely loaded. This test ensures the namespace isn't
polluted, regardless of the load state of ActiveSupport.

ActiveSupport patches Kernel to add `class_eval` but this behavior wasn't
loaded in the test environment. This created a discrepancy between test
and prod, causing tests to fail that should have passed and vice versa.
Fully loading ActiveSupport makes the test environment more accurate.