Implement a progressive rollout strategy to opt more users in the Gutenberg editor.

Phase 1: set the block editor as the default editor for new posts. We'll only target people who didn't already switched back from the block editor to Aztec

Phase 2: set the block editor as the default editor for users who already switched back to Aztec from a previous version of the app.

We'll rollout phase 1 and phase 2 progressively on randomly selected users. We'll target wpcom users first:

• phase 1 - 10% - release 13.7
• phase 1 - 30% - release 13.8
• phase 1 - 100% - release 13.9
• phase 2 - 30% - release 14.0
• phase 2 - 100% - release 14.1