So my question is, what should I add to my ca-certificates.conf file in order to get the required R3 Lets Encrypt crt/pem (see I have no idea what I'm talking about!) into my Ubuntu /usr/share/ca-certificates folder?
Retrying fetcher due to error (2/4): Bundler::Fetcher::CertificateFailureError Could not verify the SSL certificate for https://rails-assets.org/.
There is a chance you are experiencing a man-in-the-middle attack, but most likely your system doesn't have the CA certificates needed for verification. For information about OpenSSL certificates, see http://bit.ly/ruby-ssl. To connect without using SSL, edit your Gemfile sources and change 'https' to 'http'.
Nah, that's not it. It's just that LTS keeps around in the "Current" list for a long time and non-LTS not. At "Future" you can find the 23.x release. But that's indeed a good thing to notice in the future And not by accident upgrade to 21.x somehow, as it's end of life already