Move SinkBinding to v1beta1

[nachocano]

Problem
We need to promote SinkBinding to v1beta1.
Some tasks this will entail, and can probably be done in different PRs.

• v1beta1 types with UTs
• add new version to CRD
• defaulting and conversion webhook
• Update docs and examples (most in knative/docs)
• Migrate reconciler to v1beta1 with UTs
• Update e2e tests to use v1beta1
• Make v1beta the storage version. --> Decided to wait one release in order to do this
• Storage migration task. --> If we don't change the storage version, then this is not needed then for now

Persona:
Developer

Exit Criteria
SinkBinding in v1beta1

Additional context (optional)
Some example PRs
types: #2407
defaulting: #2645
conversion: #2611, #2632
reconciler: #3075

[vaikas]

Also need to run a storage version migration job after the storage has been flipped. Something like this:
https://github.com/knative/eventing/blob/master/config/pre-install/v0.16.0/storage-version-migration.yaml

[nachocano]

Also need to run a storage version migration job after the storage has been flipped. Something like this:
https://github.com/knative/eventing/blob/master/config/pre-install/v0.16.0/storage-version-migration.yaml

Thanks I wasn't aware of this, as is my first conversion/migration issue.
Will add that now.

[nachocano]

Added a separate issue for migration and flipping the storage bit, as it has to be done after 0.17 cuts.
#3615

Closing this issue then