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 #141 (comment)

[lieres]

Looks like a good idea to me.