Make ApiServerSource v1beta1 the stored version and add a migration tool

[capri-xiyue]

Problem
In 0.17, we promoted ApiServerSource to v1beta1.
This is to track that we should make v1beta1 the storage version for 0.18, and we should add a migration tool for it.
Similar to: https://github.com/knative/eventing/blob/master/config/pre-install/v0.16.0/storage-version-migration.yaml.
But need to check whether the upgrade issue for v1alpha1 apiseversource gets solved or not:
https://github.com/knative/eventing/pull/2872/files#r401917238

Persona:
Which persona is this feature for?

Exit Criteria
ApiServerSource in v1beta1 and able to migrate

Time Estimate (optional):
How many developer-days do you think this may take to resolve?

Additional context (optional)
#3611

[nachocano]

/assign @nlopezgi

[nlopezgi]

@n3wscott could you provide an update about https://github.com/knative/eventing/pull/2872/files#r401917238. I'm ready to start working on this issue but it seems something like #3895 will not be sufficient from what in can understand from the comment above? Thanks for any advice!

[nlopezgi]

I've been looking in more detail at the effort required for this issue:
There are differences between v1alpha1 and v1beta1 for ApiServerSource that make the migration tool non trivial.
controller and controllerSelector defined in ApiServerResource have been deprecated.
The documentation mentions
"Per-resource controller flag will no longer be supported in v1alpha2, please use Spec.Owner as a GKV."
"Per-resource owner refs will no longer be supported in v1alpha2, please use Spec.Owner as a GKV."

My thought is that for this task its necessary to create a tool similar to #3110 for ApiServerSource.
The tool will look for v1alpha1 ApiServerSource and convert it to a v1beta1 ApiServerSource by looking at the controller and controllerSelector fields and converting them to the appropriate field.

I currently will not have the cycles to finish this task in the next ~2weeks. Unassigning myself for now.

/unassign