Reject 0.26.x nodes from the seeds

Summary:
The 0.26.x versions are obsolete.
Also simplify the regex a bit.

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/D14053