Mark crate as unmaintained in RustSec Advisory Database

[threema-theo]

Hi,

thanks for having looked after tsify-next!

Now that it's deprecated (68a7ac8), I propose to add an entry to https://github.com/RustSec/advisory-db .

Are there any objections? If not I can go ahead and do it.

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And as another step you could archive this github repository.

[siefkenj]

@threema-theo Thanks for looking into this. Yes, please mark tsify-next as unmaintained.