microview_monswitch: Use SPDX identifier for GPL license

As the MicroView library is GPL-3.0-or-later, this is also the only
viable option for this project.

Author: raimue

src/microview_monswitch/monswitch.cpp

@@ -1,20 +1,6 @@
-/*
-MicroView Arduino Library
-Copyright (C) 2014 GeekAmmo
-
-This program is free software: you can redistribute it and/or modify
-it under the terms of the GNU General Public License as published by
-the Free Software Foundation, either version 3 of the License, or
-(at your option) any later version.
-
-This program is distributed in the hope that it will be useful,
-but WITHOUT ANY WARRANTY; without even the implied warranty of
-MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-GNU General Public License for more details.
-
-You should have received a copy of the GNU General Public License
-along with this program.  If not, see <http://www.gnu.org/licenses/>.
-*/
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Copyright (C) 2021 Rainer Müller <raimue@codingfarm.de>
+
 #include <MicroView.h>
 
 #define BUTTON0 A1