[FEATURE] Retire Build `v1alpha1` API

[adambkaplan]

Feature Tracker

This is a high level tracker for retiring the v1alpha1 API.

Child Issues

• [FEATURE] Use v1beta1 APIs Only cli#294
• [FEATURE] Stop Serving v1alpha1 API build#1686
• Operator: Remove Builds Validating Webhook On Upgrade
• Operator: Advertise "Seamless Upgrade" Capability
• Website: Remove v1alpha1 References
• Website: Document upgrade instructions for Build components