meson: Fix license field

Before this commit, the field specified "GPL2+" as license which is
wrong on several levels:

- the `license` field should specify a SPDX license identifier (which
  "GPL2+" is not)
- throughout the whole repository multiple licenses are used (not just
  GPL2 and later)
- the main library code is licensed under the LGPL, version 2.1 and
  later

Fix this by providing a proper SPDX identifier:

- The main library code is licensed under the LGPL, version 2.1 and
  later
- The tests are licensed under either the Apache (v2) license or GPL
  (v2 or later)

meson.build

@@ -1,6 +1,6 @@
 project('libsecret', 'c',
   version: '0.21.4',
-  license: 'GPL2+',
+  license: 'LGPL-2.1-or-later AND GPL-2.0-or-later AND Apache-2.0',
   meson_version: '>= 0.50',
 )