Schema - Make 'multiple' input_attr a true boolean

schema/Contact/Contact.entityType.php

@@ -232,7 +232,7 @@ return [
         'duplicate_matching',
       ],
       'input_attrs' => [
-        'multiple' => '1',
+        'multiple' => TRUE,
         'control_field' => 'contact_type',
       ],
       'pseudoconstant' => [
@@ -483,7 +483,7 @@ return [
         'duplicate_matching',
       ],
       'input_attrs' => [
-        'multiple' => '1',
+        'multiple' => TRUE,
       ],
       'pseudoconstant' => [
         'option_group_name' => 'preferred_communication_method',

schema/Contribute/ContributionPage.entityType.php

@@ -157,7 +157,7 @@ return [
       'add' => '2.1',
       'serialize' => CRM_Core_DAO::SERIALIZE_SEPARATOR_TRIMMED,
       'input_attrs' => [
-        'multiple' => '1',
+        'multiple' => TRUE,
       ],
       'pseudoconstant' => [
         'option_group_name' => 'recur_frequency_units',

schema/Core/ActionSchedule.entityType.php

@@ -471,7 +471,7 @@ return [
       'add' => '4.7',
       'serialize' => CRM_Core_DAO::SERIALIZE_SEPARATOR_TRIMMED,
       'input_attrs' => [
-        'multiple' => '1',
+        'multiple' => TRUE,
         'label' => ts('Recipients Language'),
       ],
       'pseudoconstant' => [

schema/Event/Participant.entityType.php

@@ -136,7 +136,7 @@ return [
         'duplicate_matching',
       ],
       'input_attrs' => [
-        'multiple' => '1',
+        'multiple' => TRUE,
         'label' => ts('Participant Role'),
       ],
       'pseudoconstant' => [

tests/phpunit/api/v4/Action/GetFieldsTest.php

@@ -83,6 +83,9 @@ class GetFieldsTest extends Api4TestBase implements TransactionalInterface {
     $this->assertNull($fields['id']['default_value']);
 
     $this->assertEquals(['import', 'export', 'duplicate_matching'], $fields['id']['usage']);
+
+    $this->assertSame('contact_type', $fields['contact_sub_type']['input_attrs']['control_field']);
+    $this->assertTrue($fields['contact_sub_type']['input_attrs']['multiple']);
   }
 
   public function testComponentFields(): void {