Code Review Bench PR #31483 - Refactor FXIOS-14485 FXIOS-14472 [Swift 6 Migration] Turn on Swift 6 at the Project Level, as well as Client and ClientTest targets

[tomerqodo]

Code Review Bench PR mozilla-mobile#31483

Original PR Title: Refactor FXIOS-14485 FXIOS-14472 [Swift 6 Migration] Turn on Swift 6 at the Project Level, as well as Client and ClientTest targets
Original PR Description: ## 📜 Tickets
FXIOS-14485
FXIOS-14472

💡 Description

• Turn on Swift 6 at the Project Level.
• Also Turn on Swift 6 at the Client and ClientTest target levels.
• Make BVC webView methods consistent in their completion handler annotations: @escaping @MainActor @Sendable (matching WKWebView APIs)
• Fix lingering strict concurrency errors at the project level.

No warnings as far as I can tell... let's see what Bitrise says.

📝 Checklist

• I filled in the ticket numbers and a description of my work
• I updated the PR name to follow our PR naming guidelines
• I ensured unit tests pass and wrote tests for new code
• If working on UI, I checked and implemented accessibility (Dynamic Text and VoiceOver)
• If adding telemetry, I read the data stewardship requirements and will request a data review
• If adding or modifying strings, I read the guidelines and will request a string review from l10n
• If needed, I updated documentation and added comments to complex code

Original PR URL: mozilla-mobile#31483