[MultiKueue] Promote MultiKueue API and feature gate to Beta

[mimowo]

What would you like to be added:

Promote the MultiKueue API to Beta.

Why is this needed:

For the ease of use.

The feature was first introduced in 0.6, and since then a bunch of issues have been fixed.

Completion requirements:

Issues I would like to get addressed before graduating (except from other graduation criteria mentioned in KEP):

• [ClusterQueue] Provide more details about the mis-configuration in the message for Active=False #3099
• [MultiKueue] AdmissionCheck conditions are confusing #3094

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.

[mimowo]

/cc @mwielgus @tenzen-y @alculquicondor @trasc

I think we should resolve #3094 before the gradution, but discussion is welcome.

[trasc]

/assign

[tenzen-y]

@mimowo @trasc Could we finalize #2458 before we graduate MK?
I am suspecting that the new functional requirements will be found by #2458.

[mimowo]

Thanks for the comment, I was thinking about it, but I think we don't need to block on it, here is the couple of thoughts I have:

• it is very unlikely we will be able to finalize #2458 before end of November, given the load of other workstreams. The KEP proved to be challenging, and we need a proper design for it, which I think includes decoupling Job handling and ACs.
• supporting external Jobs may will likely require new APIs, but I believe we can grow it later step by step, nothing in the current API seems would be blocking the support
• in the past we supported built-in Jobs in Kueue (initially just batch Job) as Beta API, and only later we added Job framwork and expanded and support external Jobs. I believe we can repeat the pattern here, while already in Beta.
• we are already getting user feedback that the current API is useful for JobSet, but the setup still requires many steps

EDIT: also, the support for external Jobs isn't mentioned in the KEP, so I believe we can de-couple it.