Bump version to 0.20.11

Summary: See title.

Test Plan:

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

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4850