Keep the guardian service within the PHP the plugin supports

`true` as a return type is PHP 8.2, but the plugin advertises 8.1, so the
family screen's two service calls fataled on the 8.1 test job while every
other job passed. They now return `?\WP_Error` — null on success — which
matches RegistrationGate::validate() and works on 8.1.

PHPStan was analysing against whatever PHP happened to be running (8.3 in
CI, newer locally), so `composer lint` was green on syntax the plugin
promises not to use. It is now pinned to the supported 8.1-8.3 range, which
reproduces this failure at lint time instead of three jobs later.

Co-Authored-By: Claude Opus 5 <[email protected]>

tests/Unit/Guardian/FamilyPageTest.php

@@ -182,7 +182,7 @@ class FamilyPageTest extends TestCase
             'child_dob'        => '2015-04-02',
         ];
 
-        $this->guardians->shouldReceive('updateChild')->once()->with(5, 42, 'Ada L', '2015-04-02')->andReturn(true);
+        $this->guardians->shouldReceive('updateChild')->once()->with(5, 42, 'Ada L', '2015-04-02')->andReturn(null);
 
         $captured = null;
         $this->capturingPage($captured)->maybeHandleSubmit();
@@ -194,7 +194,7 @@ class FamilyPageTest extends TestCase
     {
         $_POST = ['us_family_action' => 'remove', 'child_id' => '42'];
 
-        $this->guardians->shouldReceive('removeChild')->once()->with(5, 42)->andReturn(true);
+        $this->guardians->shouldReceive('removeChild')->once()->with(5, 42)->andReturn(null);
 
         $captured = null;
         $this->capturingPage($captured)->maybeHandleSubmit();