immutable: true|false for 0.97

[roderik]

Graph tooling 0.97.0 enforces the use of immutable, even for things where immutable is false. It would be great if the compiler could do this automatically. This way no change is needed to the open zeppelin subgraph definitions.

cfr: https://github.com/OpenZeppelin/openzeppelin-subgraphs/issues/69 cfr: https://github.com/graphprotocol/graph-tooling/pull/1994