FIX: ThemeSettingsValidator.validate_value returning wrong error (#25901)

Why this change?

Before this change, the error messages returned when validating theme
settings of typed objects was an array of array instead of just an
array.

lib/theme_settings_validator.rb

@@ -52,7 +52,7 @@ class ThemeSettingsValidator
           translation_prefix: "string",
         )
       when types[:objects]
-        errors.push(
+        errors.concat(
           ThemeSettingsObjectValidator.validate_objects(schema: opts[:schema], objects: value),
         )
       end