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>

GCDWebServer.podspec

@@ -16,6 +16,7 @@ Pod::Spec.new do |s|
   s.subspec 'Core' do |cs|
     cs.source_files = 'GCDWebServer/**/*.{h,m}'
     cs.private_header_files = "GCDWebServer/Core/GCDWebServerPrivate.h"
+    cs.frameworks = 'UniformTypeIdentifiers'
     cs.ios.library = 'z'
     cs.ios.frameworks = 'CoreServices', 'CFNetwork'
     cs.tvos.library = 'z'