message: Appstream: add content rating and change metadata license (linux)
  This changes the appdata license to CC0 from MIT (which doesn't really mean much to us) and also adds content rating because flathub started requiring these.
diff: <component type="desktop">
  <id type="desktop">com.toggl.TogglDesktop.desktop</id>
  <project_license>BSD-3-Clause</project_license>
  - <metadata_license>MIT</metadata_license>
  + <metadata_license>CC0-1.0</metadata_license>
  + <content_rating>none</content_rating>
  <name>Toggl Desktop</name>
  <summary>Simple and Intuitive Time Tracking Software</summary>
  <description>
  