Update Sparkle to use EdDSA key

While working on #824, I found this new warning in the logs emitted by Sparkle:

> DEPRECATION: Serving updates without a DSA key is now deprecated and may be removed from a future release. See Sparkle's documentation for more information.

The Sparkle docs [explain how to generate the EdDSA key](https://sparkle-project.org/documentation/#eddsa-ed25519-signatures) and also says

> https://sparkle-project.org/documentation/#eddsa-ed25519-signatures

Start with the Sparkle EdDSA/Ed25519 documentation linked in the issue, then locate the project's Sparkle configuration and update-signing entry point. Confirm how the existing DSA setup is wired before determining the required key configuration; done means the deprecation warning is gone and updates can be verified with the new signing method.

Written by the indexing model from the issue text.