GHA: add upcoming (already frozen) Fedora 39

Author: Al2Klimov

.github/workflows/linux.yml

@@ -30,6 +30,7 @@ jobs:
           - fedora:36
           - fedora:37
           - fedora:38
+          - fedora:39
           - opensuse/leap:15.3 # SLES 15.3
           - opensuse/leap:15.4 # and SLES 15.4
           - opensuse/leap:15.5 # and SLES 15.5