Add SPDX license info to the source files

Also update LICENSE copyright dates.

Author: ssilverman

LICENSE

@@ -1,6 +1,6 @@
 The Clear BSD License
 
-Copyright (c) 2019 Shawn Silverman
+Copyright (c) 2018-2023 Shawn Silverman
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without

src/SLIPStream.cpp

@@ -1,5 +1,7 @@
+// SPDX-FileCopyrightText: (c) 2018-2023 Shawn Silverman <[email protected]>
+// SPDX-License-Identifier: BSD-3-Clause-Clear
+
 // This file is part of the SLIPStream library.
-// (c) 2018-2023 Shawn Silverman
 
 #include "SLIPStream.h"
 

src/SLIPStream.h

@@ -1,7 +1,9 @@
+// SPDX-FileCopyrightText: (c) 2018-2023 Shawn Silverman <[email protected]>
+// SPDX-License-Identifier: BSD-3-Clause-Clear
+
 // Library for reading and writing SLIP using an underlying Stream object.
 // This implements [RFC 1055](https://tools.ietf.org/html/rfc1055).
 // This file is part of the SLIPStream library.
-// (c) 2018-2023 Shawn Silverman
 
 #ifndef SLIPSTREAM_H_
 #define SLIPSTREAM_H_