Update minimum supported version to 1.14
This is the version that is available in Ubuntu 18.04.
gtk-rs git master only supports GLib 2.56 or newer now, which is from the same time as GStreamer 1.14 and both are the versions from Ubuntu 18.04.
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This is the version that is available in Ubuntu 18.04.
gtk-rs git master only supports GLib 2.56 or newer now, which is from the same time as GStreamer 1.14 and both are the versions from Ubuntu 18.04.