[gnome-software] (7 commits) ...Merge branch '1225-do-not-allow-distro-upgrades-if-release-art-is-missing' into 'main'

[Milan Crha <mcrha src gnome org>]

Summary of changes:

  146b42b... data: Update the upgrade-bg.png (*)
  dde0312... gtk-style.css: Tweak upgrade-banner CSS (*)
  d9f956d... gs-plugin-fedora-pkgdb-collections: Fallback to upgrade-bg. (*)
  fbb3200... gs-plugin-fedora-pkgdb-collections: Change background image (*)
  2328155... gs-plugin-fedora-pkgdb-collections: Remove text shadow from (*)
  a1cdfbf... gs-plugin-fedora-pkgdb-collections: Get upgrade banner back (*)
  22a5d0c... Merge branch '1225-do-not-allow-distro-upgrades-if-release-

(*) This commit already existed in another branch; no separate mail sent