As many open source projects have started doing it, we're removing the
current year from the copyright notice, so that we don't need to bump
it every year.
It seems like only the first year of publication is technically
relevant for copyright notices, and even that seems to be something
that many companies stopped listing altogether (in a version controlled
codebase, the commits are a much better source of date of publication
than a hardcoded copyright statement).
We also now list Godot Engine contributors first as we're collectively
the current maintainers of the project, and we clarify that the
"exclusive" copyright of the co-founders covers the timespan before
opensourcing (their further contributions are included as part of Godot
Engine contributors).
Also fixed "cf." Frenchism - it's meant as "refer to / see".
Using this command:
```
find -name "thirdparty" -prune -o -name "*.h" -exec sed -i {} -e '/return /! s/\t\([A-Za-z0-9_]* \*[A-Za-z0-9_]*\)\;/\t\1 = nullptr;/g' \;
```
And then reviewing the changes manually to discard the ones that don't
seem correct/safe/good (notably changes to `core` unions).
