Backend: Remove `ci_builds.stage` column

Description

First iteration of Remove legacy columns from ci_builds (#271619 - closed).

Removing ci_builds.stage column will make shipping partitioning easier.

Technical Proposal

• Validate nothing is using the column
• Check space before and after drop
• Ignore the column
• Remove the column
• Update any documentation as needed.

1. Remove reads from ci_builds.stage
2. Drop Ci::EnsureStageService if possible
3. Ignore column
4. Drop column

Code we can remove 🔥

• Ci::EnsureStageService
• composite_status
• StageUpdateWorker
• Ci::Stage#update_legacy_status