Simplify shard and drain label keys

[timebertt]

What would you like to be added:

Currently, the shard and drain label keys follow this pattern:

{shard,drain}.alpha.sharding.timebertt.dev/clusterring-<hash>-<clusterring-name>

With #429, the clusterring- part is dropped, because there is no need to distinguish between different ring kinds.

Let's further simplify the key pattern to this:

{shard,drain}.alpha.sharding.timebertt.dev/<controllerring-name>

For this, we need to restrict the name of ControllerRing objects not to be longer than 63 characters.

Why is this needed:

The label key part after the / must not exceed 63 characters.
That's why the hash was added, i.e., to distinguish between ClusterRing objects with names longer than 63 characters.

This is too complicated for the fact that such long names are rather an edge case.
Let's exclude such edge cases using validation in favor of a simplified developer experience.