Update the agent pattern filter in the makeseeds.py script

Summary: Select seeds running the latest major release.

Test Plan:
Check the output from your seeds with this filter.
For reference I got 35 peers from my seeder.

Reviewers: #bitcoin_abc, deadalnix, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D4059