docs: Update license information in README and meson.build

Some of the files in `malcontent-control` are GPL-2.0+.

Use the latest SPDX identifiers in the machine-readable data in
meson.build, but use the deprecated ‘+’ form in the README since it’s a
little more human friendly.

See https://spdx.org/licenses/.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

README.md

@@ -101,7 +101,9 @@ Dependencies
 Licensing
 ---------
 
-All code in this project is licensed under LGPL-2.1+. See COPYING for more details.
+All code in the libraries in this project is licensed under LGPL-2.1+. Code in the
+`malcontent-control` application is licensed under GPL-2.0+. See `COPYING` and the
+copyright headers in individual files for more details.
 
 Bugs
 ----

meson.build

@@ -1,7 +1,7 @@
 project('malcontent', 'c',
   version : '0.6.0',
   meson_version : '>= 0.49.0',
-  license: 'LGPLv2.1+',
+  license: ['LGPL-2.1-or-later', 'GPL-2.0-or-later'],
   default_options : [
     'buildtype=debugoptimized',
     'warning_level=2',