Update License headers

[FreyJo]

• remove explicit name list from headers
• add Contributors.txt file to replace it
• removed copyright year as recommended https://hynek.me/til/copyright-years/
• removed unused CPP lint files

[nielsvd]

In addition to the inline comments. With this rework, I recommend adding a CONTRIBUTING.md file that contains at least:

Thank you for your interest to contribute to acados. All contributions are licensed under the terms of the
2-Clause BSD License. By making a contribution to acados you certify that you have the right to submit
the code that you are contributing to the project according to the Developer Certificate of Origin (DCO).