[FSSDK-13264] chore: bump native Swift SDK to 6.0.0

[muzahidul-opti]

Summary

Native Swift SDK bump for 4.0.0, after the SPM-only change in #119.

 ios/optimizely_flutter_sdk/Package.swift
-  .package(url: ".../swift-sdk.git", exact: "5.5.0")
+  .package(url: ".../swift-sdk.git", exact: "6.0.0")

Swift 6.0.0 only changes packaging: it drops CocoaPods and requires iOS 15. #119 already made both changes here. Between v5.5.0 and v6.0.0 the only file changed under Sources/ is SDKVersion.swift, so the SDK's API is unchanged. No Dart or bridge changes.

FSSDK-13264 scope:

• Android android-sdk 5.4.0: already done in chore: bump native SDKs to Swift 5.5.0 and Android 5.4.0 #120
• Xcode 27 CI runner: not needed. build_test_ios only runs flutter test (it never compiles iOS), and the testapp iOS job builds the plugin on its own runner. If that job fails here, its runner image needs updating.

.github/copilot-instructions.md updated to list the new version.

Evidence

• Before: Swift SDK 5.5.0.
  After (local, Xcode 27.0):

  flutter analyze lib test   No issues found!
  flutter test               +140: All tests passed!
  

• iOS compile against 6.0.0: covered by the integration_ios_tests / testapp iOS checks on this PR.
  I couldn't check it locally: cd example && flutter build ios --simulator already fails on master with an SPM identity mismatch (optimizely_flutter_sdk vs optimizely-flutter-sdk) when it resolves the plugin by path. That problem predates this PR.

Merge Danger

Door: two-way. Revert the one-line pin. Nothing is published until the 4.0.0 release PR merges.
Blast Radius: iOS consumers of the next release resolve swift-sdk 6.0.0. They already need iOS 15 and SPM since #119, so there's no new requirement. Android and Dart are untouched.

Issues

• FSSDK-13264

🤖 Generated with Claude Code

[Mat001]

Looks good. Approving.