Migrate exit_codes.dart to null-safety (#3128)

diff --git a/lib/src/exit_codes.dart b/lib/src/exit_codes.dart
index 27b66e6..e072ef2 100644
--- a/lib/src/exit_codes.dart
+++ b/lib/src/exit_codes.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.10
-
 /// Exit code constants.
 ///
 /// From [the BSD sysexits manpage][manpage]. Not every constant here is used,