Removes Ci::LegacyStage

What does this MR do and why?

This MR is 1/3 to completely remove Ci::LegacyStage.

Part 1 (this MR):

• Removes Ci::LegacyStage model and a few methods using it
• Removes Ci::LegacyStage presenter
• Removes the ci_stage factory that was using Ci::LegacyStage and left only ci_stage_entity
• Updates parts of the code that were using legacy stage

In Part 2, the ci_stage_entity factory will be renamed to ci_stage to be consistent. !91255 (merged)
In Part 3, we will evaluate whether it's possible to remove StageUpdateWorker and Ci::EnsureStageService. This would be an additional step towards deprecating stage related columns of ci_builds table. (WIP) !91399 (closed)
In Part 4, We will also move StageEntity to ci because that is where it should be 😇

#238586 (closed)

Impact

Removed 500 legacy LoC

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.