gnome3.webkitgtk: 2.20.2 -> 2.20.3

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

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

These checks were done:

- built on NixOS
- /nix/store/kg4axk1r8r3lw4hidrb639lvnhidkmg1-webkitgtk-2.20.3/bin/WebKitWebDriver passed the binary check.
- 1 of 1 passed binary check by having a zero exit code.
- 0 of 1 passed binary check by having the new version present in output.
- found 2.20.3 with grep in /nix/store/kg4axk1r8r3lw4hidrb639lvnhidkmg1-webkitgtk-2.20.3
- directory tree listing: https://gist.github.com/9bcce0d9c55941e792a46ccc39ce7efb
- du listing: https://gist.github.com/79054d0bb16c7cff8f636f5af11a2408