Link a recommended Dafny Server version to one version of the Dafny-VSCode plugin

[fmehta]

Currently, the latest version of the Dafny Server is installed by this plugin. In case there is a new version of the Dafny Server, the user is prompted to perform an update.

Risk: The newest version of the Dafny Server may not work well with the current version of the Dafny-VScode plugin.

Solution: Link a recommended Dafny Server version to one version of the Dafny-VSCode plugin. Installation of another version should also be possible (for early adopters or testers).

[kjx]

Given that the latest release of the dafny-server just broke things, this would be great to have fixed