Update copyright header

Despite acting as a maintainer, I believe that removing the names and
keeping only my own makes it seem like I somehow declared some of these
contributions as (relatively) insignificant, which is absolutely not
the case.

The header was originally condensed because it was getting way too big
to manage and contributor(s) were insisting on not being added there,
as their contributions 'seemed too minor'.

Adding the copyright header consistently to every file, with every
name, also came across as a burden to everyone involved.