Fix ubuntu-18.04 deprecation

CI workflow spills deprecation notice:

    CI: .github#L1
    The ubuntu-18.04 environment is deprecated, consider switching to ubuntu-20.04(ubuntu-latest), or ubuntu-22.04 instead. For more details see actions/runner-images#6002

Change is from ubuntu-18.04 to ubuntu-20.04.

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ on:
 jobs:
   build:
     name: PHP Composer
-    runs-on: 'ubuntu-18.04'
+    runs-on: 'ubuntu-20.04'
     steps:
       - name: Checkout PR
         uses: actions/checkout@v3
@@ -61,12 +61,12 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        machine: ['ubuntu-18.04']
+        machine: ['ubuntu-20.04']
         php-version: ['7.2']
         run-job: ['SH lint', 'Buildsh', 'Bash Autocompletion', 'Magento 1.9.4.5 PHP 7.2']
         experimental: [false]
         include:
-          - machine: 'ubuntu-18.04'
+          - machine: 'ubuntu-20.04'
             php-version: '7.3'
             run-job: 'OpenMage LTS 20.0.14 PHP 7.3'
             experimental: false
@@ -99,7 +99,7 @@ jobs:
 
   test-setup:
     name: Test Setup
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - {name: mysqld,shell: bash,run: 'sudo systemctl start mysql.service'}
 
@@ -136,7 +136,7 @@ jobs:
 
   codecov: # previously codecov was bound via circleci which paused for ~two years
     name: Code Coverage
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - {name: mysqld,shell: bash,run: 'sudo systemctl start mysql.service'}