Update warning to fine when flutter version file not found(#4190)

lib/src/sdk/flutter.dart

diff --git a/lib/src/sdk/flutter.dart b/lib/src/sdk/flutter.dart
index 57ed748..bb3cf12 100644
--- a/lib/src/sdk/flutter.dart
+++ b/lib/src/sdk/flutter.dart
@@ -84,7 +84,7 @@
       }
       return Version.parse(flutterVersion);
     } on IOException catch (e) {
-      warning(
+      fine(
         'Could not open flutter version file at `$flutterVersionPath`: $e\n',
       );
       return null;