Disable default branch for MR policy if default branch is unprotected

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Background

Following up on feedback from the customer, the user tries to create a project-level policy and selects default branch, the default branch is not a protected branch, and user got an error which is not clear what has happened

Solution

• figma: https://www.figma.com/file/am8lmbO4FNS5gDd7H7Y0qD/Policy-small-design-2024?type=design&node-id=6%3A17498&mode=design&t=4R4asOi3G44D6Sto-1

Project level: Disable the dropdown selection	Group level:Show a clear error message and add (protect) to dropdown selection