Build/Test Tools: Run test workflows on old branches on a schedule.

On TravisCI, old branches still receiving security updates were tested on a regular basis. This ensured tests continued to pass as time passed even if updates were not made to these branches.

On GitHub Actions, there is no interface to configure this (TravisCI had a UI), but there is a `schedule` event that can trigger workflow runs on cron that can be used to accomplish the same thing.

This introduces a workflow file that runs twice a month (on the 1st and 15th) to verify the tests within older branches.

Because the `schedule` event only runs within the primary branch, the appropriate workflows in each old branch will be triggered manually through the `workflow_dispatch` trigger using the GitHub REST API. `workflow_dispatch` will need to be added to all workflows in all old branches in order for the event to dispatch successfully.

Merges [50590] to the 5.7 branch.
See #52653.
Built from https://develop.svn.wordpress.org/branches/5.7@50591


git-svn-id: http://core.svn.wordpress.org/branches/5.7@50204 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7.1-alpha-50588';
+$wp_version = '5.7.1-alpha-50591';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.