Update copyright headers up to 2023
The logical sequel to !42 (merged) and also part of !28 ─ consisting the highest chunk of it (25,900 / 33,632 diff lines)
This update copyright header information. No code should have been changed. I assume no one will bother reviewing it, as long that it compiles?
1915 files changed, 3834 insertions(+), 1917 deletions(-)
I myself never reviewed copyright header changes unless I needed to... (There's twice insertions than deletions because it includes "ManaVerse Developers" after "ManaPlus Developers", preserving original copyright).