fix: Use correct copyright year (#4098)

* fix: Use correct copyright year

This patch corrects the copyright year included in the built `axe.js` and `axe.min.js` files. Deque Systems, Inc has held the copyright for axe-core since 2015.

* Update lib/intro.stub

Co-authored-by: Wilco Fiers <WilcoFiers@users.noreply.github.com>

---------

Co-authored-by: Wilco Fiers <WilcoFiers@users.noreply.github.com>

lib/intro.stub

@@ -1,5 +1,5 @@
 /*! axe v<%= pkg.version %>
- * Copyright (c) <%= grunt.template.today("yyyy") %> Deque Systems, Inc.
+ * Copyright (c) 2015 - <%= grunt.template.today("yyyy") %> Deque Systems, Inc.
  *
  * Your use of this Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this