Update makeseeds

Summary:
This patch:

• Adds next Bitcoin ABC release(s)
• Fixes out-of-date or missing agent strings

Test Plan:

python3 makeseeds.py < seeds_main.txt > nodes_main.txt
python3 makeseeds.py < seeds_test.txt > nodes_test.txt
git diff   # verify seed list still looks good

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7167