zbus/cargo: raise lower bound requirement on zvariant
This bumps the minimum zvariant requirement to at least 3.1.2,
in order fix build failures when using older semver-compatible
but buggy versions (e.g. 3.0.0).
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This bumps the minimum zvariant requirement to at least 3.1.2,
in order fix build failures when using older semver-compatible
but buggy versions (e.g. 3.0.0).