freeswitch: 1.6.19 -> 1.6.20

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/freeswitch/versions.

These checks were done:

- built on NixOS
- ran `/nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20/bin/freeswitch -h` got 0 exit code
- ran `/nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20/bin/fs_cli -h` got 0 exit code
- ran `/nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20/bin/fs_cli --help` got 0 exit code
- ran `/nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20/bin/fs_cli -V` and found version 1.6.20
- ran `/nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20/bin/fs_cli -v` and found version 1.6.20
- ran `/nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20/bin/fs_cli --version` and found version 1.6.20
- ran `/nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20/bin/fs_cli -h` and found version 1.6.20
- ran `/nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20/bin/fs_cli --help` and found version 1.6.20
- ran `/nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20/bin/fsxs --help` got 0 exit code
- found 1.6.20 with grep in /nix/store/l4s511s11hp68d9nask5izbjp46d38a6-freeswitch-1.6.20
- directory tree listing: https://gist.github.com/9f4955edd83c39fb3a3623049474525a