schemas: Center windows by default

Now that the requirements to do this are met, we can finally
center new windows by default.

Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2123
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1662
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/246
Closes: https://bugs.launchpad.net/ubuntu/+source/mutter/+bug/1898442
Closes: https://bugs.launchpad.net/ubuntu/+source/mutter/+bug/1298263
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4294>

data/org.gnome.mutter.gschema.xml.in

@@ -94,7 +94,7 @@
     </key>
 
     <key name="center-new-windows" type="b">
-      <default>false</default>
+      <default>true</default>
       <summary>Place new windows in the center</summary>
       <description>
         When true, the new windows will always be put in the center of the