--dry-run with a sufficiently recent version of Certbot uses the ACMEv2 staging URL regardless of whether you’re configured to use the ACMEv1 or ACMEv2 production URLs. It’s good to confirm that it works, but it doesn’t itself prove that Certbot will use the right production URL next time.
You probably don’t need to do anything with that line. It shouldn’t be doing any harm. (It’s probably not doing anything useful, either.)
Let’s Encrypt removed the TLS-SNI-01 challenge type, and added the TLS-ALPN-01 challenge type. (But it doesn’t depend on which API version you use.)
Certbot doesn’t support TLS-ALPN-01 yet (though recent versions have started work on it).
I assume your Certbot installation is using HTTP-01 now and will happily continue doing so into the future.
I assume you’re probably not using the “manual” plugin, the only one that supports both DNS-01 and HTTP-01 validation. For example, if you’re using the “webroot” plugin, it only implements HTTP-01, so the
standalone_supported_challenges setting – if it’s even applied; I’m not sure – wouldn’t change anything.
Old versions of Certbot saved default settings into the renewal configuration files, so they always had a few dozen lines of stuff you didn’t change and have never heard of. Newer versions more or less only save settings you’ve modified, but if your installation has been around a while, the files may still have some unnecessary cruft, like
standalone_supported_challenges setting was even renamed a long time ago; the old name was entirely removed at some point after 0.31.0.)