update macos image (#3838)

* update macos image

* whatsnew

Author: max-sixty

azure-pipelines.yml

@@ -38,7 +38,7 @@ jobs:
       py38:
         conda_env: py38
   pool:
-    vmImage: 'macOS-10.13'
+    vmImage: 'macOS-10.15'
   steps:
   - template: ci/azure/unit-tests.yml
 

doc/whats-new.rst

@@ -86,6 +86,8 @@ Internal Changes
 - Changed test_open_mfdataset_list_attr to only run with dask installed
   (:issue:`3777`, :pull:`3780`).
   By `Bruno Pagani <https://github.com/ArchangeGabriel>`_.
+- Updated Azure CI MacOS image, given pending removal.
+  By `Maximilian Roos <https://github.com/max-sixty>`_
 
 .. _whats-new.0.15.0: