Update makeseeds.py to support 0.28.x

Summary: This is the next minor release bump, so be prepared to accept seeds using that version.

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/D14601