Copyright: Update Authors list

All source files have an `Authors` list at the top of the file. This list only contains Volker, most of the times. Other contributors are often missing.

Options:
1. We could add everyone based on git history, but this will quickly get out of hands. `src/utils.cpp` would show 33 author lines, from a quick glance (`git log --format='format:%an' src/util.cpp | sort -u | wc -`).
1. Listing only Volker tends to become less valid over time.
1. Dropping authors altogether and listing them somewhere else instead (`src/utils.cpp` does) would also imply to drop Volker's name, which does not sound nice to me.
1. Maybe a compromise would be `Volker Fischer, The Jamulus Development Team and contributors`?

- [ ] Headers
- [ ] src/util.cpp:1929 (mentioned by @ann0see in https://github.com/jamulussoftware/jamulus/pull/2331#discussion_r797922651)

_The topic was initially raised by @pljones in https://github.com/jamulussoftware/jamulus/pull/2325#discussion_r795429539_