Drop PHP 7.0, and update Magento driver to support PHP 7.3

cli/Valet/Brew.php

@@ -17,7 +17,6 @@ class Brew
         'php@7.3',
         'php@7.2',
         'php@7.1',
-        'php@7.0',
     ];
     const BREW_DISABLE_AUTO_CLEANUP = 'HOMEBREW_NO_INSTALL_CLEANUP=1';
     const LATEST_PHP_VERSION = 'php@8.2';

cli/Valet/Drivers/Specific/Magento2ValetDriver.php

@@ -8,8 +8,12 @@ class Magento2ValetDriver extends ValetDriver
 {
     /**
      * Holds the MAGE_MODE from app/etc/config.php or $ENV.
+     *
+     * Can't be correctly typed yet because PHP 7.3.
+     *
+     * @param string|null
      */
-    private ?string $mageMode = null;
+    private /*?string*/ $mageMode = null;
 
     /**
      * Determine if the driver serves the request.