Bump version of macOS used on Azure (Qiskit#12571)

The macOS 11 runners are deprecated pending removal.  While macOS 14 is
available, it's still marked as in preview on Azure, so macOS 13 is the
current "latest" stable version.

.azure/test-macos.yml

@@ -10,7 +10,7 @@ parameters:
 jobs:
   - job: "MacOS_Tests_Python${{ replace(parameters.pythonVersion, '.', '') }}"
     displayName: "Test macOS Python ${{ parameters.pythonVersion }}"
-    pool: {vmImage: 'macOS-11'}
+    pool: {vmImage: 'macOS-13'}
 
     variables:
       QISKIT_SUPPRESS_PACKAGING_WARNINGS: Y