Bump version to 0.21.7

Summary: See title.

Test Plan:

ninja
bitcoind --version
bitcoin-cli --version
bitcoin-qt --version
bitcoin-tx -h | head
bitcoin-wallet -h | head

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6033