extensions: update in the GEX format

Our custom keys are now listed under <custom/> instead of <metadata/>,
with the same format otherwise.

This change was initially triggered by using as_component_get_custom()
in libappstream instead of as_app_get_metadata() in appstream-glib (cf.
MR !2326). It seems that libappstream doesn't have any API to get the
<metadata/> keys, and that this tag is not officially in the spec
anyway.

The custom tag on the other hand is officially in the specs (and has
been added in 2018 apparently, cf. commit 1b1ba1f61 in the appstream
repository).

See also commit Infrastructure/gimp-extensions-web@f9f497b.

Without this, all extensions were broken with MR !2326. They were parsed
and correctly listed, but no entry point to plug-ins or data were
registered.

extensions/goat-exercises/org.gimp.extension.goat-exercises.metainfo.xml.in.in

@@ -24,7 +24,7 @@
   <requires>
    <id version="2.99.0" compare="ge">org.gimp.GIMP</id>
   </requires>
-  <metadata>
+  <custom>
    <value key="GIMP::plug-in-path">@GOAT_EXERCISES@</value>
-  </metadata>
+  </custom>
 </component>