Add elastic watcher for deprecation warnings

We log deprecation warnings to the deprecation_json log stream. However, this is pull-only currently and requires developers to actively search production logs for these warnings.

Deprecation warnings can turn into hard failures if the library that raises them is upgraded. While we try to detect them in tests, they can slip past us if the test does not correctly represent how a method is used. We ran into such a case where a Ruby 3 kwargs error slipped past our test suite undetected.

This makes sure we alert on these so that we can act earlier on potential errors.