Change year to 2023

At least the following should be changed:

• Year in the copyright header
  • 2022 --> 2022, 2023
  • XXXX, 2022 --> XXXX, 2023
• license-header-check.bash

Last years there were some unanticipated changes because I did not use the build-in search/replace tool of Eclipse (see !246 (merged)).