InstanceName of plugins produces unknown field error in Kong 2.8.x

[randmonkey]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

When InstanceName is specified in KongPlugin or KongClusterPlugin, the error happens when sending configuration to Kong 2.8.x

 {"name":"invalid declarative configuration","fields":{"plugins":[{"instance_name":"unknown field"}]},"code":14,"message":"declarative config is invalid: {plugins={{instance_name=\"unknown field\"}}}"}

This makes Kong gateway stuck on updating configuration.

Expected Behavior

Valid configuration of plugins (without instance_name field) sent to Kong when Kong version is below 3.2.0.
Ref:
https://docs.konghq.com/gateway/2.8.x/admin-api/#plugin-object
https://docs.konghq.com/gateway/3.1.x/admin-api/#plugin-object
https://docs.konghq.com/gateway/3.2.x/admin-api/#plugin-object
The instance_name field is first added in Kong 3.2.x.

Steps To Reproduce

1. Create a KongPlugin foo with field InstanceName
2. Create an Ingress referencing this plugin by adding annotation konghq.com/plugins: "foo"

Kong Ingress Controller version

2.12.2

Kubernetes version

1.28.0 kind cluster

Anything else?

This does not affect KIC 3.0.x because KIC 3.x only support Kong 3.4.1 and above.

[randmonkey]

Fixed in #5250. TODO: add CHANGELOG.

[pmalek]

Would it be possible to conditionally add instance name for Kong Gateway versions older than 3.2? Would it increase the complexity in a way that it would significantly increase the maintenance costs?