Update macOS images on Azure Pipelines

The `macos-11` images were deprecated in January 2024 [1] and were
removed in June 2024 [2].

[1]: https://github.blog/changelog/2024-01-30-github-actions-macos-14-sonoma-is-now-available/
[2]: https://github.blog/changelog/2024-05-20-actions-upcoming-changes-to-github-hosted-macos-runners/

.azure-pipelines.yml

@@ -12,7 +12,7 @@ jobs:
   - job: iOS
     displayName: iOS / Xcode
     pool:
-      vmImage: macOS-12
+      vmImage: macOS-13
     steps:
       - template: ci/azure-pipelines/steps-iOS.yml
         parameters:
@@ -61,7 +61,7 @@ jobs:
   - job: macOS_Make
     displayName: macOS / Unix Makefiles
     pool:
-      vmImage: macOS-11
+      vmImage: macOS-12
     variables:
       cmakeGenerator: "Unix Makefiles"
     steps:
@@ -72,7 +72,7 @@ jobs:
   - job: macOS_Xcode
     displayName: macOS / Xcode
     pool:
-      vmImage: macOS-11
+      vmImage: macOS-12
     steps:
       - template: ci/azure-pipelines/steps-Xcode.yml
         parameters: