Remove no longer required FA1.2.1

Problem: Babylon upgrade introduced entrypoints feature that lifts some of the restrictions and makes it possible to standardize extensible interfaces. FA1.2.1 used proxy contract approach which is expensive in terms of gas, and no longer required due to the network upgrade. Leaving this standard in the repository might cause confusion, and would require us to maintain outdated recommendations.

Solution: Remove FA1.2.1 in favor of FA1.2, update the documents accordingly.

Additional cleanup:

• Unify "entrypoint" spelling across the repository;
• Removes unnecessary "manual annotations" step in build instructions for AbstractLedger and ManagedLedger.