Fix the QA 'Protected branch support' test for CE

What does this MR do?

It looks like there's no need to click on the select again to close it now. It makes sense since in CE you can only choose one option, so the select closes itself once an option is selected.

Put main logic for selecting allowed into CE protected_branches which EE calls.

For CE, the dropdown doesn't need to be closed like the EE version does.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #196841 (closed)