[ci skip] changed a 2020 to 2021 (#1801)

lol imagine being in 2020

licenseheader.txt

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2020 GeyserMC. http://geysermc.org
+ * Copyright (c) 2019-2021 GeyserMC. http://geysermc.org
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal