Bump version to 0.20.10

Summary: As per title.

Test Plan:

make
bitcoind --version
bitcoin-cli --version
bitcoin-qt --version
bitcoin-tx -h

ninja
bitcoind --version
bitcoin-cli --version
bitcoin-qt --version
bitcoin-tx -h

Reviewers: #bitcoin_abc, deadalnix, nakihito, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, nakihito, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4793