ci: replace macOS 10.15 with macOS 11 in CI pipelines

GitHub started the deprecation process for macOS 10.15 runners
on 2022-05-31, and macOS 10.15 will be completely unsupported by
2022-08-30 (if things proceed as planned).

See <actions/runner-images#5583> for
more information on the removal of the macOS 10.15 images.

This issue already hit us during the build for PR #716, because
at that time there was a scheduled brownout for macOS 10.15
builds to raise awareness of the upcoming removal.

.azure-pipelines.yml

@@ -53,9 +53,9 @@ jobs:
         parameters:
           cxxver: '14'
 
-  - job: 'macos1015_xcode11_cmake'
+  - job: 'macos11_xcode13_cmake'
     pool:
-      vmImage: 'macOS-10.15'
+      vmImage: 'macOS-11'
     steps:
       - script: which clang++ && clang++ --version
         displayName: 'Check clang'

.github/workflows/ci.yml

@@ -79,7 +79,7 @@ jobs:
             os: ubuntu-20.04
           - toolset: clang
             cxxstd: "14,17"
-            os: macos-10.15
+            os: macos-11
 
     runs-on: ${{matrix.os}}