Changeset 47198

Code Modernization: Replace dirname( __FILE__ ) calls with __DIR__ magic constant.

This avoids the performance overhead of the function call every time dirname( __FILE__ ) was used instead of __DIR__.

This commit also includes:

• Removing unnecessary parentheses from include/require statements. These are language constructs, not function calls.
• Replacing include statements for several files with require_once, for consistency:
  • wp-admin/admin-header.php
  • wp-admin/admin-footer.php
  • wp-includes/version.php

Props ayeshrajans, desrosj, valentinbora, jrf, joostdevalk, netweb.
Fixes #48082.