tracer: declare GstTracer API stable

It's been around for more than 4 years and people have built
lots of stuff on top of it, doesn't really make sense to keep
it marked as unstable. We're unlikely to change it now, and
we can always deprecate it and make a new one if needed.

This stabilises the following API:
- gst_tracer_register()
- gst_tracing_get_active_tracers()
- gst_tracing_register_hook()
- gst_tracer_record_new()
- gst_tracer_record_log()

Might also help a bit with #424

Part-of: <gstreamer/gstreamer!576>
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merge request