Fix gitlab:seed:merge_trains:project rake task

What does this MR do and why?

The DEFAULT_BRANCH constant does not exist. It probably disappeared during code review when the task was introduced. Instead we can use the project's default branch.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Run the rake task:

bin/rake gitlab:seed:merge_trains:project

This should create a project with merge trains enabled and 20 open merge requests.

MR acceptance checklist

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