Investigate whether pomerium-cli installation instructions should be updated

[kenjenkins]

Page: https://www.pomerium.com/docs/deploy/clients/pomerium-cli#packages

What's incorrect or missing

@cmo-pomerium reports a few issues when following the inline Debian instructions:

• apt-key add may need to be run as root
• writing to /etc/apt/sources.list.d/ may also require root
• apt-key outputs a deprecation warning

What's the resolution?

Should we remove the inline instructions? It looks like the Cloudsmith links show slightly different instructions.

Alternatively, let's determine if these instructions should be updated to use /etc/apt/trusted.gpg.d/ instead, per https://opensource.com/article/22/9/deprecated-linux-apt-key.