Update main net seed list to containe only nodes that support the new magic

Summary: This is necessary as we are going to use that magic by default.

Test Plan: Start a node without a peer list. Ensure it bootstraps properly.

Reviewers: freetrader, schancel, sickpig, dagurval, #bitcoin_abc

Reviewed By: sickpig, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D567