Stabilise the API

[ascopes]

Once everything is stable, we should mark the API as being stable.

This will mean enabling mutation testing (#134) on all builds in CI, and also changing all API guardian @API annotations that are currently setting the status attribute as EXPERIMENTAL so that they are STABLE instead.

INTERNAL annotations should stay how they are.

[sohel-79]

Hi as #134 is done what about this issue? Can you guide on this one i want to contribute more

[ascopes]

Hmm, you could pick this up, yeah. Think it is a good time to be doing it.

Go for it.

It is just a case of finding

@API(since = ..., status = Status.EXPERIMENTAL)

and replacing with

@API(since = ..., status = Status.STABLE)

...we want to keep anything marked as INTERNAL as it is though.