ENH: Switch Github Actions Linux environment

Switch Github Actions Linux environment version to `ubuntu-20.04`.

`ubuntu-18.04` is being deprecated and support will end by 12/1/2022:
actions/runner-images#6002

Related to recent warnings in the Azure Pipelines Linux environment
being used in the main ITK repository:
InsightSoftwareConsortium/ITK@497c3a7

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

@@ -130,9 +130,9 @@ jobs:
     strategy:
       max-parallel: 3
       matrix:
-        os: [ubuntu-18.04, windows-2019, macos-10.15]
+        os: [ubuntu-20.04, windows-2019, macos-10.15]
         include:
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             c-compiler: "gcc"
             cxx-compiler: "g++"
             cmake-build-type: "MinSizeRel"
@@ -229,7 +229,7 @@ jobs:
     strategy:
       max-parallel: 3
       matrix:
-        os: [ubuntu-18.04, windows-2019, macos-10.15]
+        os: [ubuntu-20.04, windows-2019, macos-10.15]
 
     steps:
       - uses: actions/checkout@v1
@@ -254,9 +254,9 @@ jobs:
     strategy:
       max-parallel: 3
       matrix:
-        os: [ubuntu-18.04]
+        os: [ubuntu-20.04]
         include:
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             c-compiler: "gcc"
             cxx-compiler: "g++"
             cmake-build-type: "Release"