Build/Test Tools: Remove the `workflow_run` event from the Slack notification workflow.

The `workflow_run` event was added to restore Slack notifications for older branches (5.8 and older) without having to backport any changes while alternate approaches were explored.

The workflow has been tested and refined as a reusable one in `trunk`, and this approach is superior to the `workflow_run` event in several ways.

Primarily, the `workflow_run` event results in a separate workflow run being created for sending Slack notifications after the completion of each workflow triggered by `push`. When called as a reusable workflow, this does not happen and the additional jobs are instead added to the initial workflow. This makes which jobs are sending notifications for the current workflow more clear, and reduces the amount of noise (less workflow runs overall).

The `workflow_run` event also makes some data available in different ways than `push` events. By removing it, much of the logic within the workflow can be simplified.

See #56095.
Built from https://develop.svn.wordpress.org/trunk@53591


git-svn-id: http://core.svn.wordpress.org/trunk@53179 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53589';
+$wp_version = '6.1-alpha-53591';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.