[CMAKE] Move version to the top level CMakeLists.txt

Summary:
Apart from making it easier to figure out where it is located, it will
also avoid duplicating it when using CPack.

Depends on D4027.

Test Plan:

ninja

Check the version is correct:

./src/bitcoind --version
./src/qt/bitcoin-qt --version
./src/bitcoin-cli --version
./src/bitcoin-tx -h

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4028