Build/Test Tools: Convert GitHub action workflows into reusable ones.

With a few exceptions, GitHub Actions workflows run using the version of the workflow file present in the commit SHA or Git ref for the triggering event. This is useful for maintaining different versions of a workflow file.

In the case of WordPress where there are currently 25+ branches that could potentially receive a security fix, it creates a huge maintenance burden. When 3rd party actions are updated or features are deprecated on GitHub Actions, the required changes need to be backported to all of those branches. This takes considerable time and effort.

This change converts Core’s workflow files to reusable ones. This allows the same workflow to be used for all (or most) branches, allowing the described maintenance updates to be made once in `trunk`.

To keep track of which files are reusable vs. those that are responsible for holding the strategy matrix for that branch, reusable workflows are now prefixed with `reusable-`.

Props johnbillion, swissspidy, jorbin, desrosj.
Fixes #61213.
Built from https://develop.svn.wordpress.org/trunk@58165


git-svn-id: http://core.svn.wordpress.org/trunk@57628 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.6-alpha-58164';
+$wp_version = '6.6-alpha-58165';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.