Remove years from copyright notice in files

[schmoelder]

As far as I know, we do not need the years in the copyright statement. For example, curl recently removed them: https://daniel.haxx.se/blog/2023/01/08/copyright-without-years/ So my proposal is to just get rid of the years in the copyright statement. This would avoid changing them every year.

Originally posted by @sleweke-bayer in https://github.com/modsim/CADET/issues/141#issuecomment-1958952184

[lieres]

Looks like a good idea to me.