[CICD] Change MAIN_BRANCH variable value to 'scarthgap'

The development branch 'scarthgap-dev' has been migrated to the
production one and thus CI/CD need to be changed accordingly.

.gitlab-ci.yml

@@ -11,8 +11,7 @@ variables:
   GITLAB_CI_REVISION: 7b608ad118cee3d9d2cf57f5a0be866e40081c0f
   SUBMODULE_NAME: seco-build-configurator
   VERSION_FILE: "src/version.py"
-  # MAIN_BRANCH must be set to "scarthgap" once the initial development is completed
-  MAIN_BRANCH: "scarthgap-dev"
+  MAIN_BRANCH: "scarthgap"
 
 # Jobs specific for the 'seco-build-configurator' repo
 # that are not defined in the 'gitlab-ci' one