Bump version to 0.21.12
Summary: See title.
Test Plan:
./bitcoind --version | head -n1 ./bitcoin-cli --version | head -n1 ./qt/bitcoin-qt --version | head -n1 ./bitcoin-tx --help | head -n1 ./seeder/bitcoin-seeder --version | head -n1 ./bitcoin-wallet --help | head -n1
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D6926