Update release process to anticipate on unreleased Cardano node

[jpraynaud]

Why

There is a circular dependency between the Mithril and Cardano node release processes:

• Mithril currently needs a Cardano node pre-release to prepare support for it
• Including the Mithril signer binary in the Cardano node bundle requires a Mithril distribution pre-release to be ready before the Cardano pre-release is created

This chicken-and-egg problem blocks both teams and prevents Mithril from proactively preparing support for upcoming Cardano node versions.

What

Decouple the Mithril release process from the Cardano node release cycle so that Mithril can build, test, and prepare distributions against unreleased Cardano node versions independently.

How

• Simplify the release process by skipping the creation of the -pre pre-release (update ADR)
• Create usable artifacts from unreleased Cardano node versions with nix:
  • Binary
  • Docker image
• Support an unreleased Cardano node version in the end to end tests in the CI
• Support an unreleased Cardano node version in the infrastructure