ci: migrate to Xcode 26.0 and macos-26 runners

Author: coenttb

.github/workflows/ci.yml

@@ -18,10 +18,10 @@ jobs:
   # Tests use @Test and #expect which are not available in Swift 5.x
   macos-swift60:
     name: macOS (Swift 6.0)
-    runs-on: macos-15
+    runs-on: macos-26
     strategy:
       matrix:
-        xcode: ['16.0']
+        xcode: ['26.0']
         config: ['debug', 'release']
     steps:
       - uses: actions/checkout@v5
@@ -48,10 +48,10 @@ jobs:
 
   macos-swift62:
     name: macOS (Swift 6.2)
-    runs-on: macos-15
+    runs-on: macos-26
     strategy:
       matrix:
-        xcode: ['16.2']
+        xcode: ['26.0']
         config: ['debug', 'release']
     steps:
       - uses: actions/checkout@v5
@@ -120,7 +120,7 @@ jobs:
 
   readme:
     name: README Code Examples
-    runs-on: macos-15
+    runs-on: macos-26
     steps:
       - uses: actions/checkout@v5
 
@@ -136,7 +136,7 @@ jobs:
 
   documentation:
     name: Documentation
-    runs-on: macos-15
+    runs-on: macos-26
     steps:
       - uses: actions/checkout@v5