Linux packages: removed apt-key usage to import gpg keys on Debian/Ubuntu. apt-key is deprecated since 2017 and will be removed after Debian 11 and Ubuntu 22.04. All distributions since at least 2013 support storing trusted keys in /etc/apt/trusted.gpg.d/ instead, so using it should be safe. A special workaround is added for Ubuntu 16.04 shipping gnupg 2.1, which is not compatible with the CLI invocation we use.

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