Drop GVariant support

[zeenix]

As I expressed in #1125:

• the whole gvariant support is getting too much to handle for me, especially mixed with zbus etc.
• I've long abandoned the plans of using gvariant as a dbus2 protocol (which was its original intention).
• gvariant crate exists and not only it is used by ostree folks, it actually lives in the ostree space and seems to be the officially blessed implementation.

So I'm inclined to drop this support in the zvariant 6.0.

[zeenix]

• gvariant crate exists and not only it is used by ostree folks, it actually lives in the ostree space and seems to be the officially blessed implementation.

A bit more context about this: I originally added gvariant support in hopes that ostree folks will use it, since gvariant provides a nice API but seems they went for the gvariant crate, even though (IMO) it has a non-intuitive strange API that is not serde-compatible.

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Anyway, another option here would be to split the gvariant support into a separate crate. However, I would not have the time and energy to do that. So if someone wants to take that up, please go ahead and let me me here.