Replace Lint tab with Validate tab in the pipeline editor

What does this MR do and why?

For #337282 (closed)

This adds the simulate_pipeline feature flag and replaces the Lint tab of the pipeline editor with the Validate tab. The UI is static for now. In future MRs, we will add the simulate pipeline feature and other UI functionality to it.

No changelog trailers added since this is under a dev feature flag.

Feature Breakdown

UX review was done in !88630 (closed) and we've got approval, but because the MR ended up being quite big I am breaking the it down further into two parts so it's easier to give it a frontend review 😬

frontend	MR
Introduce feature flag to replace Lint tab with Validate tab in the pipeline editor	👈 you are here
Add popover and NEW badge to the Validate tab	TODO
Add simulation functionality to the Validate tab	TODO
Roll out feature flag	#364257 (closed)

Screenshots or screen recordings

Before (Lint Tab)	After (Validate Tab)

How to set up and validate locally

1. Enable the simulate_pipeline feature flag (rails c > Feature.enable(:simulate_pipeline))
2. Go to the pipeline editor (CI/CD > Editor)
3. The Lint tab should be replaced with the Validate tab.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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