License headers: use SPDX-FileCopyrightText for source/

There are still some files that need to be manually updated due to
missing copyright dates.

source/blender/freestyle/intern/python/UnaryFunction0D/BPy_UnaryFunction0DFloat.cpp

@@ -1,4 +1,6 @@
-/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* SPDX-FileCopyrightText: 2008-2023 Blender Foundation
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
 
 /** \file
  * \ingroup freestyle