Update default branch in divergence graph

What does this MR do?

This updates the default branch for the divergence graph to whatever is set on the project, instead of hard-coding the value.

This MR is part of a bigger effort to Update default initial branch name in Frontend code.

Screenshots (strongly suggested)

No visual changes are expected

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because this is a developer-facing change.
• Code review guidelines
• Merge request performance guidelines

Availability and Testing

• Tested in all supported browsers

Related to #324100