Fix PHPStan undefined variables in menu.php and network.php

[huzaifaalmesbah]

This fixes a few PHPStan "variable might not be defined" warnings..
Changes:

• src/wp-admin/includes/menu.php: Added the global declaration for $menu, $submenu, and $compat.
• src/wp-admin/includes/network.php: Initialized the $message and $message_class variables to prevent them from being undefined when the condition isn't met.

Trac: https://core.trac.wordpress.org/ticket/64238

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[noruzzamans]

Great catch on the PHPStan warnings!

Quick question:
since wp-admin/menu.php (line 440) loads includes/menu.php via require, both files share the same scope. So $menu, $submenu, and $compat should already be accessible without global.
Do you think PHPStan is flagging this because it analyzes the file in isolation? Just curious about the reasoning. The network.php fix looks solid!