Change the current status of argocd-image-updater to production-ready and release 1.0

[RmStorm]

There is a big warning on the front page of the documentation that argocd-image-updater is not ready for critical production workloads and should be considered experimental with lots of breaking changes. However looking through the releases the previous warning for breaking changes is 2.5 years old. It occured in 0.12.0. I think this mean people will ignore the warning and will use argocd-image-updater as a production ready project. Based on this survey by the argocd project a full 15% of users use argocd-image-updater. I can't be sure but I'm quite confident that this means it's used in a lot of production environments.

I think it would be right and proper for argocd-image-updater to accept this responsibility and move the status into general availability and release 1.0. Scary off course! But also necessary I think. You already have good testing and all-round seem in a pretty good place? The main thing is maybe a pre-release regime where people can opt-in to experimental releases.

[chengfang]

Please see #857.