[actions] exclude Xcode 26 beta 6

This version of Xcode includes SDKs that don’t support backdeployment of `Span` and friends. This is fixed in Xcode 26 17A321 released on September 9th.

Author: glessard

.github/workflows/pull_request.yml

@@ -11,7 +11,7 @@ jobs:
     with:
       linux_exclude_swift_versions: '[{"swift_version": "5.8"}, {"swift_version": "5.9"}, {"swift_version": "5.10"}]'
       windows_exclude_swift_versions: '[{"swift_version": "5.9"}, {"swift_version": "5.10"}]'
-      macos_exclude_xcode_versions: '[{"xcode_version": "16.0"}, {"xcode_version": "16.1"}]'
+      macos_exclude_xcode_versions: '[{"xcode_version": "16.0"}, {"xcode_version": "16.1"}, {"xcode_version": "26.b6"}]'
       enable_macos_checks: true
       enable_wasm_sdk_build: true