workflows/CI: Use ubuntu-20.04 instead of ubuntu-latest

GitHub has updated ubuntu-latest to use Ubuntu 24.04, and the version
of CMake shipped with it breaks googletest due to incompatibility with
CMake versions < 3.5, while googletest specifies a
cmake_minimum_required version of 2.6.2.

Explicitly specify the Ubuntu version as 20.04 instead of using
ubuntu-latest on the workflows jobs.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>

Author: kamalesh-babulal

.github/workflows/continuous-integration.yml

@@ -14,7 +14,7 @@ jobs:
   differential-shellcheck:
     name: Differential ShellCheck
     if: github.event_name == 'pull_request'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
 
     permissions:
       contents: read
@@ -33,7 +33,7 @@ jobs:
 
   flake8-lint:
     name: Lint
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Install flake8 dependencies
         run: |
@@ -52,7 +52,7 @@ jobs:
 
   checkpatch:
     name: Checkpatch Review
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     if: github.event_name == 'pull_request'
 
     permissions:
@@ -75,7 +75,7 @@ jobs:
     name: Doxygen
     # Only run Doxygen against the main branch
     if: github.ref_name == 'main'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
 
     steps:
     - uses: actions/checkout@v4
@@ -96,7 +96,7 @@ jobs:
 
   unittests:
     name: Unit Tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
 
     steps:
     - uses: actions/checkout@v4
@@ -422,7 +422,7 @@ jobs:
     if: ${{ always() }}
     needs: [unittests, functionaltestsv1, functionaltestsv1v2, functionaltestsv2,
             nosystemd]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
     - name: Finalize code coverage results
       uses: coverallsapp/github-action@master