Drop CI support for Xcode 16.3 and 16.4

They don't work, and main is targeting Swift 6.3 anyways, where we don't plan to support anything older than the latest shipping Xcode.

Author: jakepetroules

.github/workflows/pull_request.yml

@@ -23,7 +23,7 @@ jobs:
       enable_linux_static_sdk_build: true
       linux_static_sdk_build_command: SWIFTBUILD_STATIC_LINK=1 LLBUILD_STATIC_LINK=1 swift build
       enable_macos_checks: true
-      macos_exclude_xcode_versions: "[{\"xcode_version\": \"16.2\"}]"
+      macos_exclude_xcode_versions: "[{\"xcode_version\": \"16.3\"}, {\"xcode_version\": \"16.4\"}]"
       macos_build_command:
         swift test &&
         /usr/bin/xcrun xcodebuild -workspace . -scheme SwiftBuild-Package -destination generic/platform=iOS