Update to cardano-api 8.29 + plutus 1.15

[locallycompact]

We would like to integrate early with SanchoNet which requires cardano-node > 8.1, which probably requires update to cardano-api from our side. Best version from Core tech feedback is 8.6 which is still unreleased.

[ghost]

@locallycompact Could you elaborate on why we want to do this now in the description of the issue?

[locallycompact]

In general we want this because we can't do it in an emergency - if we discover a fault in one of our transitives and we are too far behind HEAD. This feels like a meta question on why do we upgrade at all.

[ghost]

Yes, it's definitely a meta question and something we should define a strategy for. I am sympathetic to the idea of frequent and regular upgrades in order to not fall behind, but there might be impediments here and there because of what's actually supported in deployed networks.

[ghost]

Just suggesting there might be more urgent things to address, #1113 for example