Rotate security GPG key

Summary: Best practice.

Test Plan: gpg --recv-keys 5442AB0B9178E0D1567479B471A3ED7ECF82C6A7

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2582