(MODULES-11367) Update Ubuntu GitHub Action runner

GitHub is deprecating Ubuntu 18.04 runners on April 1, 2023. This
commit switches all Ubuntu 18.04 runners used in GitHub Actions to
Ubuntu 20.04.

Author: mhashizume

.github/workflows/daily_unit_tests_with_nightly_puppet_gem.yaml

@@ -10,15 +10,15 @@ jobs:
     name: ${{ matrix.os_type }} / Puppet${{ matrix.puppet_version }} gem / Ruby ${{ matrix.ruby }}
     strategy:
       matrix:
-        os: [ 'ubuntu-18.04', 'macos-latest', 'windows-2019' ]
+        os: [ 'ubuntu-20.04', 'macos-latest', 'windows-2019' ]
         puppet_version: [ 6, 7 ]
         include:
           - puppet_version: 6
             ruby: 2.5
           - puppet_version: 7
             ruby: 2.7
 
-          - os: 'ubuntu-18.04'
+          - os: 'ubuntu-20.04'
             os_type: 'Linux'
             env_set_cmd: 'export '
             gem_file: 'puppet-latest.gem'

.github/workflows/static_code_analysis.yaml

@@ -15,7 +15,7 @@ jobs:
       ruby_version: 2.6
       extra_checks: check:symlinks check:git_ignore check:dot_underscore check:test_file
 
-    runs-on: 'ubuntu-18.04'
+    runs-on: 'ubuntu-20.04'
     steps:
       - name: Checkout current PR code
         uses: actions/checkout@v2

.github/workflows/unit_tests_with_nightly_puppet_gem.yaml

@@ -12,15 +12,15 @@ jobs:
     name: ${{ matrix.os_type }} / Puppet${{ matrix.puppet_version }} gem / Ruby ${{ matrix.ruby }}
     strategy:
       matrix:
-        os: [ 'ubuntu-18.04', 'macos-latest', 'windows-2019' ]
+        os: [ 'ubuntu-20.04', 'macos-latest', 'windows-2019' ]
         puppet_version: [ 6, 7 ]
         include:
           - puppet_version: 6
             ruby: 2.5
           - puppet_version: 7
             ruby: 2.7
 
-          - os: 'ubuntu-18.04'
+          - os: 'ubuntu-20.04'
             os_type: 'Linux'
             env_set_cmd: 'export '
             gem_file: 'puppet-latest.gem'

.github/workflows/unit_tests_with_released_puppet_gem.yaml

@@ -12,15 +12,15 @@ jobs:
     name: ${{ matrix.os_type }} / Puppet${{ matrix.puppet_version }} gem / Ruby ${{ matrix.ruby }}
     strategy:
       matrix:
-        os: [ 'ubuntu-18.04', 'macos-latest', 'windows-2019' ]
+        os: [ 'ubuntu-20.04', 'macos-latest', 'windows-2019' ]
         puppet_version: [ 6, 7 ]
         include:
           - puppet_version: 6
             ruby: 2.5
           - puppet_version: 7
             ruby: 2.7
 
-          - os: 'ubuntu-18.04'
+          - os: 'ubuntu-20.04'
             os_type: 'Linux'
           - os: 'macos-latest'
             os_type: 'macOS'