Model version is not updated

[jgadsden]

Describe the bug:

The version of threat model is stored in the model JSON file. It is never updated even though the model is saved with a later version of Threat Dragon. The model version is unchanged from when the model is created or imported.

Expected behaviour:

If the model is saved with different version of threat dragon then the model version should be updated.

Environment:

• Version: 2.0.0 onwards
• Platform: any of Web App or Desktop App
• OS: any of MacOS / Windows / Linux
• Browser: any for example: chrome, firefox, safari

To Reproduce:

open a model from a previous 2.x.x version of Threat Dragon and save it. Observe the versionhas not been updated

Any additional context, screenshots, etc: