Coding Standards: Replace PHP alias join() with implode().

In [52190], some new instances of the `join()` alias were introduced in the test suite.

Using the canonical function name for PHP functions is strongly recommended, as aliases may be deprecated or removed without (much) warning.

This replaces the new `join()` uses with the canonical `implode()` function name.

Follow-up to [46182], [49193], [49805], [52190].

Props david.binda, mukesh27.
Fixes #59389.
Built from https://develop.svn.wordpress.org/trunk@56616


git-svn-id: https://core.svn.wordpress.org/trunk@56128 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-alpha-56615';
+$wp_version = '6.4-alpha-56616';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.