gitlab-ci: stay on Fedora 39 until it is no longer supported

We can rely on the debian:testing job to test the bleeding edge, and it is
a lot easier to troubleshoot.

The Fedora job is a lot harder to troubleshoot than the Debian-based jobs,
and they are often quite bleeding edge.  Currently, there is a change to
either Python or an image processing lib (Pillow?) that now compresses PNGs
differently than all previous releases.  That breaks the tests based on
processing images and checking the SHA-256 matches.

70e7e720b9
fdroidserver!669

.gitlab-ci.yml

@@ -285,9 +285,7 @@ black:
     - black --check --diff --color $CI_PROJECT_DIR
 
 fedora_latest:
-  image: fedora:latest
-  only:
-    - master@fdroid/fdroidserver
+  image: fedora:39  # support ends on 2024-11-12
   script:
     # tricks to hopefully make runs more reliable
     - echo "timeout=600" >> /etc/dnf/dnf.conf