Pass the gitlab-ci update-transiflex only on the main branches

Requirements

Pass the gitlab-ci update-transiflex only on the main branches

Descriptive title for this enhancement

Pass the gitlab-ci update-transiflex only on the main branches

Actual behavior

Actually we pass the update-transiflex job for each MR

Expected behavior

Pass update-transiflex only when we finish a new feature / fixes when it's merged in main branches

Step by step description of new behaviour

Change the gitlab-ci file to only execute update-transiflex job when we do actions on the specified main branches.