Update default bundle in generated host.json once bundle V3 is finalized

_This is a tracking item for action to take once the V3 bundle exists in the main bundle ID._

If host config is not present, the host generates a default `host.json` file. This is used in a few places, notably the portal editing experience. Today this uses the V2 bundle:

https://github.com/Azure/azure-functions-host/blob/35913e7c2285cd5774a4647bbb84fe5caa8a5834/src/WebJobs.Script/ScriptConstants.cs#L183

Once the V3 bundle is finalized, we should update it to use V3: `"[3.*, 4.0.0)"`

Check src/WebJobs.Script/ScriptConstants.cs at the referenced line and confirm that bundle V3 has been finalized in the main bundle ID. Once it is available, update the generated default host.json bundle range to "[3.*, 4.0.0)"; the issue is done when this default no longer uses V2.

Written by the indexing model from the issue text.