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