ci: adds validate_merge_request_source_branch job

.gitlab-ci.yml

 include:
   - project: 'quiqqer/semantic-release'
     file: '/.gitlab-ci.yml'
+validate_merge_request_source_branch:
+  script:
+    - echo "Job runs to validate merge request source branch"
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event' && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "master"
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event' && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "main"
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event' && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME =~ /^(\d\.)?\d.x$/