[docs] Remove outdated bridge and null state references from AppState docs

[dr-thca]

Summary

• Removes outdated "bridge" reference from AppState.currentState documentation
• Removes mention of currentState being null at launch, which is no longer the case with the new architecture

Context

Fixes #4689

Since React Native 0.76, the new architecture (TurboModules) is the default. AppState.currentState is now populated synchronously via getConstants() during module construction, so it is never null when user code accesses it. The old documentation referenced the "bridge" and a transient null state that no longer applies.

The TypeScript types already declare currentState as non-nullable (AppStateStatus), so this change aligns the prose with the types and the runtime behavior.

Changes

Two sentences updated across 8 files (docs/appstate.md + versioned docs for 0.77-0.83):

1. Basic Usage section: Removed "However, currentState will be null at launch while AppState retrieves it over the bridge."
2. Example description: Removed ", and the null state will happen only momentarily"

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