Update license year for all packages

Fixes #20887

license.md

 The MIT License (MIT)
 
-Copyright (c) 2020 Vercel, Inc.
+Copyright (c) 2021 Vercel, Inc.
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

packages/next-codemod/license.md

 The MIT License (MIT)
 
-Copyright (c) 2020 Vercel, Inc.
+Copyright (c) 2021 Vercel, Inc.
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

packages/next-mdx/license.md

 The MIT License (MIT)
 
-Copyright (c) 2020 Vercel, Inc.
+Copyright (c) 2021 Vercel, Inc.
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

packages/next/license.md

 The MIT License (MIT)
 
-Copyright (c) 2020 Vercel, Inc.
+Copyright (c) 2021 Vercel, Inc.
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal