Build/Test: Prevent using unsupported NPM versions.

Using NPM 7+ currently causes a number of issues. In order to improve developer experience, we should prevent a developer from going down the road of using incompatible engines.

See also: WordPress/gutenberg#29204 and WordPress/gutenberg#23600.

Props rcorrales.
Fixes #56547.


Built from https://develop.svn.wordpress.org/trunk@54124

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-54123';
+$wp_version = '6.1-alpha-54124';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.