Update copyright comments

Co-authored-by: Eksperimental <eksperimental@autistici.org>

Author: Papipo

lib/mix/lib/mix/gleam.ex

@@ -1,6 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: 2021 The Elixir Team
-# SPDX-FileCopyrightText: 2012 Plataformatec
 
 defmodule Mix.Gleam do
   # Version that introduced `gleam export package-information` command

lib/mix/test/mix/gleam_test.exs

@@ -1,6 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: 2021 The Elixir Team
-# SPDX-FileCopyrightText: 2012 Plataformatec
 
 Code.require_file("../test_helper.exs", __DIR__)