Amend license header to include contributors and apply to FML subprojects (#8525)

After internal discussion, it was decided that we need to include
"contributors" to the license header. This avoids claiming that the
Java source files are under the exclusive copyright ownership of Forge
LLC, which excludes contributors that still hold copyright ownership
over their contributions (but is licensed under the LGPLv2.1 as stated
in the Forge CLA).

src/main/java/net/minecraftforge/server/ServerLifecycleHooks.java

@@ -1,5 +1,5 @@
 /*
- * Minecraft Forge - Forge Development LLC
+ * Copyright (c) Forge Development LLC and contributors
  * SPDX-License-Identifier: LGPL-2.1-only
  */