swiften: 3.0beta2 -> 4.0

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

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

These checks were done:

- built on NixOS
- ran `/nix/store/pfiq18ki0dzdd0zsbvv04sppr4f28fl5-swiften-4.0/bin/swiften-config -h` got 0 exit code
- ran `/nix/store/pfiq18ki0dzdd0zsbvv04sppr4f28fl5-swiften-4.0/bin/swiften-config --help` got 0 exit code
- ran `/nix/store/pfiq18ki0dzdd0zsbvv04sppr4f28fl5-swiften-4.0/bin/swiften-config help` got 0 exit code
- ran `/nix/store/pfiq18ki0dzdd0zsbvv04sppr4f28fl5-swiften-4.0/bin/swiften-config --version` and found version 4.0
- found 4.0 with grep in /nix/store/pfiq18ki0dzdd0zsbvv04sppr4f28fl5-swiften-4.0
- directory tree listing: https://gist.github.com/bf18dc85d27b684315737f90db4a6b64