Update seeds

Summary: See title. Note that these seeds still include 0.19.x nodes because the number of high-uptime 0.20.x nodes is still too low.

Test Plan:
make check

TOTAL=$(cat nodes_main.txt | wc -l); NUM=$(./test-seeds.sh < nodes_main.txt | wc -l); echo "scale=3; $NUM / $TOTAL" | bc
Mainnet, percent of nodes with connectivity before this patch: 83.0%
Mainnet, percent of nodes with connectivity after this patch: 88.1%

TOTAL=$(cat nodes_test.txt | wc -l); NUM=$(./test-seeds.sh --testnet < nodes_test.txt | wc -l); echo "scale=3; $NUM / $TOTAL" | bc
Testnet, percent of nodes with connectivity before this patch: 75.7%
Testnet, percent of nodes with connectivity after this patch: 86.6%

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4233