codegen: Bump minimum 'graph-ts' version required

[evaporei]

This is required after graphprotocol/graph-ts#253, otherwise subgraph developers will experience errors like these:

ERROR TS2339: Property 'displayData' does not exist on type '~lib/@graphprotocol/graph-ts/common/value/Value'. Entities of type [removed contract name] must have an ID of type String but the id '${id.displayData()}' is of type ${id.displayKind()} ~~~ in generated/schema.ts(1036,102) ERROR TS2339: Property 'displayKind' does not exist on type '~lib/@graphprotocol/graph-ts/common/value/Value'. Entities of type [removed contract name] must have an ID of type String but the id '${id.displayData()}' is of type ${id.displayKind()} ~~~ in generated/schema.ts(1036,134)

Methods like displayData are available only in graph-ts versions greater than or equal to 0.26.0

[evaporei]

The CI is failing, I'm taking a look into why 🕵️‍♀️

Edit: I figured out why, it's because we clone the example-subgraph and that wasn't up to date, well, just fixed it 👷‍♀️

[lutter]

LGTM!