Update the suggested key type in the contributing file

Summary:
The ssh-rsa public key signature algorith has been deprecated by openssh (see https://lwn.net/Articles/821544/), and it might lead to authenticatio failure if the client does not support the new rsa signature algo alternatives.

ED25519 support is live for a while and creates shorter keys, which might be useful for users with no copy/paste facility (like with some VNC clients), so suggest that instead.

Test Plan: Try the instructions

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11158