boehmgc: revert "8.0.6 -> 8.2.0", experimental release
boehmgc: revert "8.0.6 -> 8.2.0", experimental release

Revert also fixes build guile-3. Without it the build fails as:

  error: builder for '/nix/store/v3vkr9vaxg278mdi6cv2a0jaqw8mn3xp-guile-3.0.7.drv' failed with exit code 2;
       last 10 log lines:
       >                  from alist.c:31:
       > /nix/store/2dzi3ls093ffivpp6ykhb3wh33ngyaww-boehm-gc-8.2.0-dev/include/gc/gc.h:1784:11: fatal error: gc_pthread_redirects.h: No such file or directory
       >  1784 | # include "gc_pthread_redirects.h"
       >       |           ^~~~~~~~~~~~~~~~~~~~~~~~

Upstream fixed it in master only.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs