meson: let's make unified cgroupsv2 the default

Prompted by @evverx' comments:

https://github.com/systemd/systemd/pull/10161#pullrequestreview-158327715

NEWS

@@ -43,6 +43,16 @@ CHANGES WITH 243 in spe:
         * Memory protection directives can now take a value of zero, allowing
           explicit opting out of a default value propagated by an ancestor.
 
+        * systemd now defaults to the "unified" cgroup hierarchy setup during
+          build-time, i.e. -Ddefault-hierarchy=unified is now the build-time
+          default. Previously, -Ddefault-hierarchy=hybrid was the default. This
+          change reflects the fact that cgroupsv2 support has matured
+          substantially in both systemd and in the kernel, and is clearly the
+          way forward. Downstream production distributions might want to
+          continue to use -Ddefault-hierarchy=hybrid (or even =legacy) for
+          their builds as unfortunately the popular container managers have not
+          caught up with the kernel API changes.
+
           …
 
 CHANGES WITH 242:

meson_options.txt

@@ -156,7 +156,7 @@ option('fallback-hostname', type : 'string', value : 'localhost',
 option('compat-gateway-hostname', type : 'boolean', value : 'false',
        description : 'allow "gateway" as the symbolic name for default gateway')
 option('default-hierarchy', type : 'combo',
-       choices : ['legacy', 'hybrid', 'unified'], value : 'hybrid',
+       choices : ['legacy', 'hybrid', 'unified'], value : 'unified',
        description : 'default cgroup hierarchy')
 option('default-net-naming-scheme', type : 'combo',
        choices : ['latest', 'v238', 'v239', 'v240'],