Bump version to 0.20.8

Summary: I moved some release-notes that was missing a rebase from D4511...

Test Plan:

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

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

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4530