build: Change upcoming version to 40.alpha2

I messed up and released 40.alpha at the same time as 3.38.2, when it's
supposed to be in January. In order to re-align with the schedule, change
the upcoming version to 40.alpha2 so we don't have to skip a release and
will be back on track in time of 40.beta.

meson.build

@@ -1,5 +1,5 @@
 project('gnome-shell-extensions',
-  version: '40.beta',
+  version: '40.alpha2',
   meson_version: '>= 0.44.0',
   license: 'GPL2+'
 )