ENH: Switch Github Actions macOS environment

Switch Github Actions macOS environment version to `macos-11`.

`macOS-10.15` is being deprecated and support will end by 8/30/2022:
actions/runner-images#5583

Related to recent warnings in the Azure Pipelines macOS environment
being used in the main ITK repository:
InsightSoftwareConsortium/ITK@e5595de

Author: jhlegarreta

.github/workflows/build-test-publish.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       max-parallel: 3
       matrix:
-        os: [ubuntu-20.04, windows-2019, macos-10.15]
+        os: [ubuntu-20.04, windows-2019, macos-11]
         include:
           - os: ubuntu-20.04
             c-compiler: "gcc"
@@ -21,7 +21,7 @@ jobs:
             c-compiler: "cl.exe"
             cxx-compiler: "cl.exe"
             cmake-build-type: "Release"
-          - os: macos-10.15
+          - os: macos-11
             c-compiler: "clang"
             cxx-compiler: "clang++"
             cmake-build-type: "MinSizeRel"
@@ -130,7 +130,7 @@ jobs:
     strategy:
       max-parallel: 3
       matrix:
-        os: [ubuntu-20.04, windows-2019, macos-10.15]
+        os: [ubuntu-20.04, windows-2019, macos-11]
         include:
           - os: ubuntu-20.04
             c-compiler: "gcc"
@@ -140,7 +140,7 @@ jobs:
             c-compiler: "cl.exe"
             cxx-compiler: "cl.exe"
             cmake-build-type: "Release"
-          - os: macos-10.15
+          - os: macos-11
             c-compiler: "clang"
             cxx-compiler: "clang++"
             cmake-build-type: "MinSizeRel"
@@ -229,7 +229,7 @@ jobs:
     strategy:
       max-parallel: 3
       matrix:
-        os: [ubuntu-20.04, windows-2019, macos-10.15]
+        os: [ubuntu-20.04, windows-2019, macos-11]
 
     steps:
       - uses: actions/checkout@v1