cppzmq: 4.2.1 -> 4.2.3
cppzmq: 4.2.1 -> 4.2.3

Semi-automatic update. These checks were performed:

- built on NixOS
- found 4.2.3 with grep in /nix/store/vxdd7gcb6daclw3qj1qvm3b6mqpkvn1m-cppzmq-4.2.3
- found 4.2.3 in filename of file in /nix/store/vxdd7gcb6daclw3qj1qvm3b6mqpkvn1m-cppzmq-4.2.3

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