Update default branch name in pipelines editor and job specs

What does this MR do?

Updates the default branch name in pipelines specs

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related to #327686 (closed)