Change version to 0.21.0

Summary: We're going to skip 0.20.13 since we're pushing back the release a day to match the feature code freeze date of Feb 15th.

Test Plan:

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

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5289