Remove code related to Ubuntu 18.04. (#226)

.github/workflows/build-python-packages.yml

@@ -15,7 +15,7 @@ on:
       PLATFORMS:
         description: 'Platforms for execution in "os" or "os_arch" format (arch is "x64" by default)'
         required: true
-        default: 'ubuntu-18.04,ubuntu-20.04,ubuntu-22.04,macos-11,windows-2019_x64,windows-2019_x86'
+        default: 'ubuntu-20.04,ubuntu-22.04,macos-11,windows-2019_x64,windows-2019_x86'
   pull_request:
     paths-ignore:
     - 'versions-manifest.json'
@@ -39,7 +39,7 @@ jobs:
       - name: Generate execution matrix
         id: generate-matrix
         run: |
-          [String[]]$configurations = "${{ inputs.platforms || 'ubuntu-18.04,ubuntu-20.04,ubuntu-22.04,macos-11,windows-2019_x64,windows-2019_x86' }}".Split(",").Trim()
+          [String[]]$configurations = "${{ inputs.platforms || 'ubuntu-20.04,ubuntu-22.04,macos-11,windows-2019_x64,windows-2019_x86' }}".Split(",").Trim()
           $matrix = @()
 
           foreach ($configuration in $configurations) {

.github/workflows/releases-validation.yml

@@ -14,8 +14,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-latest, windows-latest, ubuntu-18.04, ubuntu-20.04]
-        python: [3.5.4, 3.6.7, 3.7.5, 3.8.1]
+        os: [macos-latest, windows-latest, ubuntu-20.04, ubuntu-22.04]
+        python: [3.9.12, 3.10.8, 3.11.10]
     steps:
     - name: setup-python ${{ matrix.python }}
       uses: actions/setup-python@v4

.github/workflows/test-python-version.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-latest, windows-latest, ubuntu-18.04, ubuntu-20.04] 
+        os: [macos-latest, windows-latest, ubuntu-20.04, ubuntu-22.04] 
     steps:
     - name: Setup Python ${{ github.event.inputs.version }}
       uses: actions/setup-python@main