appdata: Use rdns developer ID

[yakushabb]

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>

More information: https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/#name-summary-and-developer-name