certbot: Use ECDSA (P-256) certificates, not RSA

certbot switched to ECDSA by default about two years ago, following
[recommended practices][1].

We are currently using RSA with 4096 bits, which is extremely slow to
sign. Using ECDSA should give us a nice speedup.

[1]: https://wiki.mozilla.org/Security/Server_Side_TLS#Intermediate_compatibility_.28recommended.29