fixes broken CI; bump macOS version to macos-11 (#20098)

* bump macOS image on Azure CI to macos-11

##[warning]The macOS-10.15 environment is deprecated, consider switching to macos-11(macos-latest), macos-12 instead. For more details see https://github.com/actions/virtual-environments/issues/5583

* fix CI error

.github/workflows/ci_docs.yml

@@ -40,7 +40,7 @@ jobs:
           - target: windows
             os: windows-2019
           - target: osx
-            os: macos-10.15
+            os: macos-11
 
     name: ${{ matrix.target }}
     runs-on: ${{ matrix.os }}

.github/workflows/ci_packages.yml

@@ -6,7 +6,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-20.04, macos-10.15]
+        os: [ubuntu-20.04, macos-11]
         cpu: [amd64]
         batch: ["allowed_failures", "0_3", "1_3", "2_3"] # list of `index_num`
     name: '${{ matrix.os }} (batch: ${{ matrix.batch }})'