Forms: Use button class directly on buttons for consistent display

Adding the `button` mixin into `input[type="submit"] caused conflicts with the other button classes (`.button-primary`) due to specificity. The button classes should be used on all buttons instead. This also updates which button on the Manage Pledge form is the "primary" action, since it's easy to skip the Update button when it's not primary.

plugins/wporg-5ftf/views/modal-request-manage-link.php

@@ -35,6 +35,7 @@ defined( 'WPINC' ) || die();
 
 			<input
 				type="submit"
+				class="button"
 				name="get_manage_pledge_link"
 				value="<?php esc_attr_e( 'Submit', 'wporg-5ftf' ); ?>"
 			/>