Use Distro.gcc_version for GCC 15 overlay gating Replace the hand-maintained list of distros affected by the GCC 15 default-`-std=gnu23` change (which breaks OCaml < 5.1) with a check against the gcc version exposed by ocaml-dockerfile 8.4.0. The overlay is now applied whenever the distro ships gcc >= 15, so new distro releases (e.g. Fedora 44, gcc 16.1) pick it up automatically and distros that don't actually ship gcc 15 (Ubuntu 25.04, gcc 14.2; openSUSE Leap 15.6, gcc 7.5) no longer get an unnecessary patch repo. Also bumps the opam-repository pin in the Dockerfile to a commit that includes dockerfile/dockerfile-opam 8.4.0.