appdata: Add release tag

The appdata validation is now more strict and has a
mandatory requirement of the "release" tag [1]

[1] https://github.com/hughsie/appstream-glib/pull/296
4 jobs for <span class="ref-name">uajain/add-release-tag</span> in 25 minutes and 5 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #260012
flatpak

00:21:08

 
  Review
passed #260075
review

00:03:57

failed #260014
allowed to fail manual
stop_review

failed #260013
review

00:04:47

 
Name Stage Failure
failed
stop_review Review There has been a missing dependency failure
No job trace