Bump version to 0.21.8

Summary: per title

Test Plan:

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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6104