[FSSDK-13225] chore: prepare release 6.0.0

[muzahidul-opti]

Summary

Prepare for Release 6.0.0: major release for the Swift Package Manager migration.

• Bump OPTIMIZELYSDKVERSION to 6.0.0
• Add 6.0.0 CHANGELOG section covering the breaking changes from [FSSDK-11920] Migrate Swift SDK distribution, CI, and releases to SPM on Xcode 27 #658:
  • CocoaPods support removed; SDK is distributed via SPM only
  • Minimum platforms raised to iOS 15.0, tvOS 15.0, watchOS 9.0, macOS 12.0

Test plan

• All tests passing on CI
• Release workflow validation confirms the ## 6.0.0 CHANGELOG section exists
• No new functionality, version bump only

Issues

• FSSDK-13225

🤖 Generated with Claude Code

[jaeopt]

LGTM

[datadog-optimizely-experimentation]

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[coveralls]

Coverage is 93.929% — prepare-release-6.0.0 into master. No base build found for master.