libgap: 4.8.3 -> 4.8.6

Semi-automatic update. These checks were performed:

- built on NixOS
- found 4.8.6 with grep in /nix/store/05va3jfhvjfxri7311n92l8dgh3l0f0n-libgap-4.8.6
- found 4.8.6 in filename of file in /nix/store/05va3jfhvjfxri7311n92l8dgh3l0f0n-libgap-4.8.6

cc "@raskin"