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Fix #20

Also:
- use the same patterns as gstreamer-rs and gst-plugins-rs ci.
- remove unused cross build template
- update from Rust 1.36 to 1.40

Update minimum Rust version to 1.36, GStreamer to 1.16.1 and explicitly use Debian buster docker images

It should depend on the highest API version we support.

All dependencies now also are compiled with the same version feature as
the crate itself, allowing for more caching of build results by cargo.

The ARM/Windows cross-builds were not too useful on their own and we
should rather set up proper CI for them.