Release/6.0.0

[w-goog]

Prepares for the 6.0.0 release: version bump and changelog. This release updates
AppAuth to 3.0.0 and raises the minimum deployment targets, both source-breaking,
so SemVer requires a major bump.

Here are the public-facing changes:

• Xcode 27 + AppAuth 3.0.0 Support #280: AppAuth updated to 3.0.0, and minimums raised to iOS 15.0, macOS 12.0,
  tvOS 15.0, watchOS 9.0 and swift-tools-version 5.7
• Update GTMAppAuth's dependencies before the next major version bump #284: GTMSessionFetcher dependency widened to allow 4.x and 5.x (issue The gtm-session-fetcher dependency is locked to an old version #273)
• Migrate sample apps to UIKit scene-based lifecycle #278: iOS example migrated to the UIScene lifecycle, now iOS 15+
• Update iOS example to use xcconfig-based client configuration #279: iOS example switched to xcconfig-based client configuration
• Support swift 6 #267: Swift 6 compile fixes (issue Swift 6 support #258)

Two example-app fixes found during release testing are also included:

• The macOS CocoaPods example still declared a 10.12 deployment target, which Xcode 27 refuses to build.
• The iOS CocoaPods example goofed at launch because Update iOS example to use xcconfig-based client configuration #279 wired the xcconfig into the SPM project only.

These changes are also included, though they are internal:

• Update macos runner to macos-13 #266, Use macos-latest instead of hardcoded version number #274 and Restrict workflow token permissions + remove dead docs workflow #282 (CI runners and workflow token permissions)
• The Gemfile is regenerated for CocoaPods 1.17.0.