Deprecate ::assertWaitOnAjaxRequest() in favour of ::assertExpectedAjaxRequest()

Problem/Motivation

Follow-up from #3316274: Stabilize FunctionalJavascript testing AJAX: add ::assertExpectedAjaxRequest(). In that issue we added a new assertExpectedAjaxRequest() method, but didn't deprecate the old one.

Steps to reproduce

Proposed resolution

Convert all calls to assertWaitOnAjaxRequest() to assertExpectedAjaxRequest().

Add @deprecated and @trigger_error(....... E_USER_DEPRECATED) to the old method for removal in Drupal 11.

See https://www.drupal.org/about/core/policies/core-change-policies/drupal-d... for the format.

Remaining tasks

Convert to an MR
Add the deprecation notice

User interface changes

API changes

Data model changes

Release notes snippet