Add support for DSE 6.9

[adejanovski]

DSE 6.9 will go GA soon and we need to support it in cass-operator.
As part of this, we need to modify the kubebuilder validation regex to allow 6.9.x and figure out where the 6.9 images will be published to update the image config appropriately.
The simple solution being to publish the 6.9 images in datastax/dse-mgmtapi-6_8 despite the presence of 6_8 in the name of the repo.

Definition of Done

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1. 6.9 is a valid version for DSE images and can be scheduled with cass-operator