ceph: Fix build with GCC 13 by using fmt_8 -> fmt_9. Fixes #281027

pkgs/top-level/all-packages.nix

@@ -6809,8 +6809,14 @@ with pkgs;
 
   libceph = ceph.lib;
   inherit (callPackages ../tools/filesystems/ceph {
-    lua = lua5_4;
-    fmt = fmt_8;
+    lua = lua5_4; # Ceph currently requires >= 5.3
+
+    # To see which `fmt` version Ceph upstream recommends, check its `src/fmt` submodule.
+    #
+    # Ceph does not currently build with `fmt_10`; see https://github.com/NixOS/nixpkgs/issues/281027#issuecomment-1899128557
+    # If we want to switch for that before upstream fixes it, use this patch:
+    # https://github.com/NixOS/nixpkgs/pull/281858#issuecomment-1899648638
+    fmt = fmt_9;
   })
     ceph
     ceph-client;