chore: complete Swift 6 strict concurrency compatibility

[NandanPrabhu]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

Apps that depend on JWTDecode with strict Swift concurrency enabled should build successfully. The library sources already carried full Sendable conformance (added in #329), but the build configuration never compiled them in Swift 6 language mode — so nothing was enforcing that property, and regressions could land unnoticed.

No public API changes.

SWIFT_VERSION 5.0 → 6.0 across all 20 Xcode project configurations

This is the consumer-facing gap. Carthage consumers compile the sources through JWTDecode.xcodeproj, so the library was being built in Swift 5 language mode regardless of the Sendable annotations on the types.

Removed the .swiftLanguageMode(.v5) override on the SPM test target

With swift-tools-version:6.0 the target now inherits Swift 6 mode, so the tests compile under the same language mode as the library rather than opting out of it.

Removed the now-invalid @retroactive on the JWTDecodeError: Equatable test conformance

@retroactive only applies to conformances declared outside the type's own module; JWTDecodeError is in the same package, so this is an error under Swift 6. It was the one real compile error in the repo, and it was only hidden by the .v5 pin above.

📎 References

• Builds on feat: add full Sendable conformance to JWT types #329, which added the Sendable conformances this PR makes the build system actually enforce.
• chore: bump minimum deployment targets #376 (chore: bump minimum deployment targets) also targets master and edits the same project.pbxproj build configuration blocks (*_DEPLOYMENT_TARGET there, SWIFT_VERSION here), so whichever merges second will likely need a rebase.

🎯 Testing

The existing 58 tests now execute in Swift 6 language mode rather than Swift 5, which is the main test-side coverage change — the assertions are unchanged, but they now run under the concurrency checking this PR enables.

Verified locally:

• All five Xcode schemes — iOS, macOS, tvOS (58 tests each), watchOS and visionOS (build; no test target in the project)
• swift test — 58 tests
• pod lib lint — passes on all six platforms
• Complete strict concurrency with warnings-as-errors, and library-evolution mode — both clean, zero warnings

Downstream consumer check. Since the goal is stated in terms of consumer apps, I verified against a separate SPM package built with -strict-concurrency=complete -warnings-as-errors in Swift 6 mode, exercising the cases a strict-concurrency app would hit:

• Passing a decoded JWT into MainActor.run and Task.detached
• Carrying a Claim across an isolation boundary
• Storing a JWT in a @MainActor-isolated class
• Binding JWT, Claim, body, and header to a <T: Sendable> generic
• Catching JWTDecodeError across an isolation boundary

All compile with no warnings.