Just find them out by yourself. SSLLabs also shows the IPs of the server quite well…
Here is the “v1” server I found here:
https://www.ssllabs.com/ssltest/analyze.html?d=https%3A%2F%2Facme-v01.api.letsencrypt.org&latest
And here the staging/testing server:
https://www.ssllabs.com/ssltest/analyze.html?d=acme-staging.api.letsencrypt.org&latest
BTW when I’m already running an SSLLabs scan: Your HSTS header there is possibly a bit short (< 180 days) so SSLLabs complains about this.
Anyway I also think this is a nice thing for adding to the docs too.