Drop the rehearsal switch and catch the actions up to Node 24

[thorstenalpers]

What changed

• The dry-run option is gone. Triggering Release now always builds, signs and publishes; the "Build and sign, publish nothing" rehearsal path, its input and the artifact-upload steps that only served it are removed. The guard that refuses a version whose tag already exists stays.
• Actions bumped off Node 20. actions/checkout v4→v7, actions/setup-node v4→v7, actions/cache v4→v6, actions/upload-artifact v4→v7, softprops/action-gh-release v2→v3 — the versions the deprecation warning on the last CI run was about, each taken to its current major.

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