ci: fix Xcode version in readme-validation and package-validation jobs

- Update Xcode_16.0.app to Xcode_26.0.app in standalone jobs
- These jobs were missed in the initial migration

Author: coenttb

.github/workflows/ci.yml

@@ -107,7 +107,7 @@ jobs:
       - uses: actions/checkout@v5
 
       - name: Select Xcode
-        run: sudo xcode-select -s /Applications/Xcode_16.0.app
+        run: sudo xcode-select -s /Applications/Xcode_26.0.app
 
       - name: Validate Package.swift
         run: swift package dump-package
@@ -125,7 +125,7 @@ jobs:
       - uses: actions/checkout@v5
 
       - name: Select Xcode
-        run: sudo xcode-select -s /Applications/Xcode_16.0.app
+        run: sudo xcode-select -s /Applications/Xcode_26.0.app
 
       - name: Run README verification tests
         run: swift test --filter ReadmeVerificationTests