Bump Apple platform workflows to Xcode 16.3/Swift 6.1 to fix xcodebuild

The Swift Package Collection signing certificate used by Xcode 15.4
expired 10 hours ago and broke any workflows that used xcodebuild (for
SwiftCrossUI's CI at least). It's probably a good that that all
platforms are getting tested against the same Swift version now anyway.

Author: stackotter

.github/workflows/build-test-and-docs.yml

@@ -15,6 +15,8 @@ jobs:
     steps:
     - name: Force Xcode 15.4
       run: sudo xcode-select -switch /Applications/Xcode_15.4.app
+    - name: Force Xcode 16.3 (Swift 6.1)
+      run: sudo xcode-select -switch /Applications/Xcode_16.3.app
 
     - name: Swift version
       run: swift --version
@@ -70,6 +72,8 @@ jobs:
     steps:
     - name: Force Xcode 15.4
       run: sudo xcode-select -switch /Applications/Xcode_15.4.app
+    - name: Force Xcode 16.3 (Swift 6.1)
+      run: sudo xcode-select -switch /Applications/Xcode_16.3.app
 
     - name: Swift version
       run: swift --version