Graduate Object Retention Policy feature gate to Beta

[mimowo]

What would you like to be added:

Graduate the ObjectRetentionPolicy to Beta.

The feature is opt-in anyway by the configMap configuration so I think we can graduate it to Beta already.

Why is this needed:

For the ease of use of the feature.

[mimowo]

cc @tenzen-y @mwysokin @mbobrovskyi @mykysha

[mykysha]

/assign