Cleanup graviton activation

Summary:
Move the graviton activation to use a block height and convert the
activation tests to be feature tests.

Depends on D4782.

Progress towards T653: Convert activation tests to feature tests.

Test Plan:
Run IBD.

ninja check
./test/functional/test_runner.py abc-minimaldata-activation abc-schnorrmultisig-activation

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, deadalnix, markblundeberg

Maniphest Tasks: T653

Differential Revision: https://reviews.bitcoinabc.org/D4642