Don't include private-address in the default Scenario database when mocking Juju 4

EDIT: see the first reply for the new scope of this issue

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The `private-address` key is not included in the default relation settings (databag) as of Juju 4.0, so we should not include it in the mock one either, if the mock Juju version is 4.0+.

The default contents are currently set when a `Relation` (etc) object is created. We don't have access to the relevant `Context` object at that time (which is where the Juju version is), so we'll have to figure out what the best way of handling this is.