Support renamed `core.openapi.json` in 0.77.0

[edd]

For reasons related to documentation, the file:
https://github.com/vegaprotocol/rest-clients/blob/master/scripts/get-latest-specs.js#L16

is now:
https://github.com/vegaprotocol/documentation/blob/main/specs/v0.77.0/transaction.openapi.json

This is true for all releases from 0.77.0 onwards, but not 0.76.0.