Make pages_domains.project_id not nullable

A short history:

• originally the field was not nullable
• then we tried to introduce "serverless" product category via pages, and it required domains not attached to the project
• so the field has become nullable
• the "serverless" category long has been abandoned, and we removed most of it's code
• but pages domain in the database still have nullable project_id

TLDR: it's a technical debt that needs to be removed, and you need a migration for this:

• remove all pages domains with project_id null: !166123 (merged) - %17.5
• make the field non-nullable again: !166705 (merged) - %17.5

Reference: !144667 (comment 1771902970)