Update .gitlab-ci.yml file

.gitlab-ci.yml

@@ -32,15 +32,6 @@ stages:          # List of stages for jobs, and their order of execution
   - ctest -j$(nproc)
 
 ############ JOBS ############
-installation-test-xenial:
-  stage: install
-  cache: []
-  image: ubuntu:16.04
-  script:
-    - *script_gcc5_definition
-    - *script_install_definition
-    - *script_test_definition
-
 installation-test-bionic:
   stage: install
   cache: []
@@ -60,7 +51,7 @@ installation-test-focal:
 lint-test-job:
   stage: test
   cache: []
-  image: ubuntu:16.04
+  image: ubuntu:20.04
   script:
     - echo "Linting code... This will take about 10 seconds."
     - sleep 10