meson: install into /usr/share/appdata, not /usr/share/metainfo

Despite the latter is recommended by the most recent spec, not all the tooling
seems to have been updated. The autotools build system also uses the old one.
Let's not switch that inconsistently and revert back to the old path for now.

(cherry picked from commit 7734c464)

meson.build

@@ -376,7 +376,7 @@ custom_target(
   output: appdata,
   command: intltool_xml_cmd,
   install: true,
-  install_dir: join_paths(nma_datadir, 'metainfo')
+  install_dir: join_paths(nma_datadir, 'appdata')
 )
 
 schema_conf = configuration_data()