feat: add GITLAB_OA_OLDREV environment variable for merge requests

[julian-piehl]

Adds the environment variable GITLAB_OA_OLDREV to Merge Request Pipelines.

Allows users to check if there are code changes using their own pipeline and not the plugin settings.

Testing done

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
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