Migrate MIME-type lookup to UTType (UniformTypeIdentifiers)

Replace the deprecated CoreServices UTI calls
(UTTypeCreatePreferredIdentifierForTag / UTTypeCopyPreferredTagWithClass)
in GCDWebServerGetMimeTypeForExtension with the modern
[UTType typeWithFilenameExtension:].preferredMIMEType, removing the
-Wdeprecated-declarations pragma. Behavior is unchanged: known
extensions map to the same MIME type, unknown ones fall back to the
default.

The UniformTypeIdentifiers framework (macOS 11+ / iOS 14+ / tvOS 14+,
within the project's deployment targets) is now linked in all three
build systems: Xcode (OTHER_LDFLAGS), CocoaPods (Core subspec
frameworks), and SwiftPM (linkedFramework).

Verified: Run-Tests.sh, swift build, and pod lib lint all pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Package.swift

@@ -27,6 +27,7 @@ let package = Package(
       ],
       linkerSettings: [
         .linkedLibrary("z"),
+        .linkedFramework("UniformTypeIdentifiers"),
         .linkedFramework("CoreServices", .when(platforms: [.iOS, .tvOS])),
         .linkedFramework("CFNetwork", .when(platforms: [.iOS, .tvOS])),
         .linkedFramework("SystemConfiguration", .when(platforms: [.macOS])),