Downgrade to warning the new MoodleInternalNotNeeded error

When the new detection was planned at #152, it was agreed
to make it a warning, not an error.

Instead, I missed that completely and implemented it as error.

So, this simple PR just downgrades the check to warning, as
originally planned.

moodle/Sniffs/Files/MoodleInternalSniff.php

@@ -122,7 +122,7 @@ public function process(File $file, $pointer) {
 
         // Having MOODLE_INTERNAL, not having side effects and not having multiple artifacts, error.
         if ($hasMoodleInternal && !$hasSideEffects && !$hasMultipleArtifacts) {
-            $file->addError('Unexpected MOODLE_INTERNAL check. No side effects or multiple artifacts detected.',
+            $file->addWarning('Unexpected MOODLE_INTERNAL check. No side effects or multiple artifacts detected.',
                 $pointer, 'MoodleInternalNotNeeded');
             return;
         }

moodle/tests/files_moodleinternal_test.php

@@ -174,10 +174,10 @@ public function test_moodle_files_moodleinternal_unexpected() {
         $this->set_sniff('moodle.Files.MoodleInternal');
         $this->set_fixture(__DIR__ . '/fixtures/files/moodleinternal/unexpected.php');
 
-        $this->set_errors([
+        $this->set_errors([]);
+        $this->set_warnings([
             17 => 'MoodleInternalNotNeeded'
         ]);
-        $this->set_warnings([]);
 
         $this->verify_cs_results();
     }