sysupdate: rename isEnabled to enabled

The "isEnabled" name was changed from "enabled" post v261 by accident
(because a6f7ded859 replaced json
formatting via table formatter to a native json generator which was
introduced for varlink, which changed the names). THis was later made
official in #43603, but it should rather have changed things back to the
original name, in particular as the underling .feature file field
actually is called "Enabled=" and not "IsEnabled=".

We really should take care to generate the same names for Varlink JSON
data as for the data returned by the systemd-sysupdate cli.
a6f7ded859 ensured that, so that's great.
But at the same time I think the varlink names should be adjusted to the
old names, since varlink support for this was just added in this cycle.

Given that v262 is not tagged yet, let's now revert to the old name.

man/org.freedesktop.sysupdate1.xml

@@ -333,7 +333,7 @@ node /org/freedesktop/sysupdate1/target/host {
         </varlistentry>
 
         <varlistentry>
-          <term><literal>isEnabled</literal></term>
+          <term><literal>enabled</literal></term>
           <listitem><para>A boolean indicating whether this feature is enabled.</para></listitem>
         </varlistentry>