[electrum] always test the latest Bitcoin ABC and fulcrum in functional tests

Summary:
This way we will know sooner if fulcrum and Bitcoin ABC become incompatible.

And fix CONTRIBUTING.md

Test Plan:
Check that the latest tag matches the most recent docker for both Bitcoin ABC and fulcrum

pip3 install -r contrib/requirements/requirements-regtest.txt
pytest electrumabc/tests/regtest`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14273