Bump the minimum block count for makeseeds.py

Summary: The blockchain has grown since the last update, this will filter out old ABC versions without using the regex.

Test Plan: There is no easy way to test the behavior without a full seeder infra, but this is trivial enough.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15854