meson: Bump minimal meson version to 1.7

We're already requiring a much newer version than we previously reported
anyway, since we were using for example `gnome.post_install()` which is
only available from Meson 0.57 and later.

So let's bump this to a somewhat more modern version which is available
in most distros.

meson.build

@@ -1,7 +1,7 @@
 project('geary', [ 'vala', 'c' ],
   version: '46.0',
   license: 'LGPL-2.1-or-later',
-  meson_version: '>= 0.59',
+  meson_version: '>= 1.7',
 )
 
 # Determine the type of build