appdata: Add developer ID

Flathub requires a developer tag and developer ID. Also Appstream decided to use rdns structure for developer ID.

> A developer tag with a name child tag must be present.
> Only one developer tag is allowed and the name tag also must be present only once in untranslated form.

```
<developer id="tld.vendor">
  <name>Developer name</name>
</developer>
```
Source: https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/#name-summary-and-developer-name

data/com.github.geigi.cozy.appdata.xml

@@ -7,8 +7,8 @@
   <launchable type="desktop-id">com.github.geigi.cozy.desktop</launchable>
   <translation type="gettext">com.github.geigi.cozy</translation>
   <!-- developer_name tag deprecated with Appstream 1.0 -->
-  <developer id="github.com">
   <developer_name translate="no">Julian Geywitz</developer_name>
+  <developer id="sh.cozy">
     <name translate="no">Julian Geywitz</name>
   </developer>
   <update_contact>[email protected]</update_contact>