chore(license): Relicense library + service to MPL-2.0

As the sole author of the library and service, I elect to relicense the
code that I have written from GPL-3.0 to MPL-2.0. This is to preserve
copyleft-ability while permitting linking of pop-launcher's libraries.

The launcher binary and its plugins shall remain as GPL-3.0, as they are
separate binaries with no need for linking to a frontend.

src/codec.rs

@@ -1,5 +1,5 @@
-// SPDX-License-Identifier: GPL-3.0-only
-// Copyright © 2021 System76
+// Copyright 2021 System76 <info@system76.com>
+// SPDX-License-Identifier: MPL-2.0
 
 use blocking::Unblock;
 use futures_lite::{AsyncBufReadExt, AsyncRead, Stream, StreamExt};