Deprecate KongIngress CRD entirely

[programmer04]

Is there an existing issue for this?

• I have searched the existing issues

Problem Statement

Standard Ingress and gateway-api are feature reach enough to express everything, thus KongIngress can be entirely deprecated.

Proposed Solution

No response

Additional information

Required to be resolved/resolves/obsoletes:

• Policy attachment for Upstreams that will supersede KongIngress.spec.upstreams #3174
• Disable support for deprecated KongIngress fields #3018
• Invalid KongIngress caused service downtime #4210

Acceptance Criteria

• KongIngress.(Service|Route) fields are not allowed to be set
• KongIngress.Upstream field is deprecated (a warning is returned from the admission webhook)

[mflendrich]

Blocked by the issues mentioned in

Required to be resolved/resolves/obsoletes

[czeslavo]

@mheap Is this still valid that we want to remove KongIngress in KIC 3.0 completely? KongIngress.Upstream hasn't been deprecated and dropping that might be unexpected for users. We only had it being planned in the Using KongIngress guide:

The upstream section of KongIngress will be replaced with a new resource, but this is still in development and upstream is not officially deprecated yet.

[czeslavo]

After a short discussion I've updated the ACs to reflect what we want to actually achieve by resolving this issue.