includes: update copyright year in footer

Author: flit

_includes/footer.html

@@ -2,7 +2,7 @@
   <div class="row row-cols-1 row-cols-sm-2 row-cols-md-4 pt-4">
     <div class="col-3">
       <p class="small">
-          Copyright &copy; 2021-2022 PyOCD Authors.
+          Copyright &copy; 2021-2023 PyOCD Authors.
       </p>
       <p class="small">
         Site and docs are <a href="https://creativecommons.org/licenses/by/4.0/">CC BY 4.0</a>.<br/>