fix: normalize ACF migration results (#2128)

Cast mixed migrator results to the handler's declared boolean contract and
return false for unsupported field definitions.

This prevents successful ACF record IDs from causing strict return type errors
while preserving existing boolean outcomes.

Author: jeanfredrik

Modularity/source/php/Upgrade/Migrators/Module/AcfModuleMigrationHandler.php

@@ -38,8 +38,10 @@ private function migrateField(string $oldFieldName, $newField): bool
 
         if (is_string($newField)) {
             $migrator = new AcfModuleFieldMigrator($newField, $oldFieldValue, $this->moduleId);
-            return $migrator->migrate();
+            return (bool) $migrator->migrate();
         }
+
+        return false;
     }
 
     private function migrateFieldByType(string $oldFieldName, $oldFieldValue, array $newField): bool
@@ -55,7 +57,7 @@ private function migrateFieldByType(string $oldFieldName, $oldFieldValue, array
             $migrator = new $class($newField, $oldFieldValue, $this->moduleId);
         }
 
-        return isset($migrator) ? $migrator->migrate() : false;
+        return isset($migrator) ? (bool) $migrator->migrate() : false;
     }
 
     private function isRemoveFieldMigration($newField)