Move PingSource to v1beta2

[capri-xiyue]

Problem
We need to promote PingSource to v1beta2.
Some tasks this will entail, and can probably be done in different PRs.

• - v1beta2 types with UTs
• - add new version to CRD
• - defaulting and conversion webhook
• - Update docs and examples (most in knative/docs)
• - Migrate reconciler to v1beta2 with UTs
• - Update e2e tests to use v1beta2
• - Create issue to make v1beta2 the storage version and add it in migration tool in next release

Persona:
developer

Exit Criteria
PingSource in v1

Examples: #3606 and #3627

[lionelvillard]

/cc @cr22rc @duglin

[lionelvillard]

blocked by #4206

[lberk]

@capri-xiyue were you taking this one on (given the discussions on #4206)?

[capri-xiyue]

@capri-xiyue were you taking this one on (given the discussions on #4206)?

@eclipselu will take this one on and @lionelvillard will help.

[eclipselu]

/assign @eclipselu