Validation of versions ignores channel

[janiskemper]

/kind bug

What steps did you take and what happened:
We validate that the versions are correctly defined in our clusterstack webhook. However, we ignore the channel. We allow custom versions even if the channel is "stable". We have to make an additional check in our webhook that the channels of the versions match the channel specified in the clusterstack.

What did you expect to happen:
We have to validate for the stable channel that the channel of the versions is actually stable. For the custom channel we can allow anything that is a valid version and don't have to change anything.

We need to have tests that verify this and make sure it actually works as expected.