Add documentation for running a Preinstall job for upgrading Broker storage versions to v1beta1

[vaikas]

What is changing? (Please include as many details as possible.)

eventing v1alpha1 API is being removed in 0.16 and prior to upgrade users must run a job that upgrades storage versions from v1alpha1 to v1beta1.

How will this impact our users?

They must run the upgrade tool before upgrade.

In what release will this take happen (to the best of your knowledge)?

v0.16

Context

Link to associated PRs or issues from other repos here.

1. Add a 'Preinstall' storage migrator for Brokers. eventing#3261

Additional info

[vaikas]

This will be a two step process:

• Install https://github.com/knative/eventing/blob/master/config/pre-install/v0.16.0/broker.yaml
• Then run the rest of the yamls.

Reason being that unless you first run the migrator job, there will not be any migration done, we must first bump the storage version to v1beta1, then run the job so that everything gets migrated.

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