Bump Xcode versions in GitHub actions. (#1593)

Bump to Xcode 26.3 and 26.4 Beta.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

.github/workflows/main_using_main.yml

@@ -22,6 +22,6 @@ jobs:
       linux_static_sdk_versions: '["nightly-main"]'
       windows_swift_versions: '["nightly-main"]'
       enable_macos_checks: true
-      macos_xcode_versions: '["26.2"]'
+      macos_xcode_versions: '["swift_6.3"]'
       enable_wasm_sdk_build: true
       wasm_sdk_versions: '["nightly-main"]'

.github/workflows/main_using_release.yml

@@ -22,6 +22,6 @@ jobs:
       linux_static_sdk_versions: '["nightly-6.3"]'
       windows_swift_versions: '["nightly-6.3"]'
       enable_macos_checks: true
-      macos_xcode_versions: '["26.2"]'
+      macos_xcode_versions: '["swift_6.3"]'
       enable_wasm_sdk_build: true
       wasm_sdk_versions: '["nightly-6.3"]'

.github/workflows/pull_request.yml

@@ -12,6 +12,13 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  list-xcodes:
+    name: List available Xcode versions
+    runs-on: [self-hosted, macos]
+    steps:
+      - name: List available Xcode versions
+        run: |
+            ls -ld /Applications/*Xcode*
   tests:
     name: Test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@0.0.7
@@ -21,9 +28,9 @@ jobs:
       linux_static_sdk_versions: '["nightly-main"]'
       windows_swift_versions: '["nightly-main", "nightly-6.3"]'
       enable_macos_checks: true
-      macos_xcode_versions: '["26.2"]'
+      macos_xcode_versions: '["swift_6.3"]'
       enable_ios_checks: true
-      ios_host_xcode_versions: '["26.2"]'
+      ios_host_xcode_versions: '["swift_6.3"]'
       enable_wasm_sdk_build: true
       wasm_sdk_versions: '["nightly-main", "nightly-6.3"]'
       enable_android_sdk_build: true