Added the multiSelect option to stop event propagation when clicking on the dropdown

What does this MR do?

Looking closer at the shouldPropagate function of gl_dropdown.js we can see that propagation is only stopped if the dropdown has been instantiated with the multiSelect option. 


A simple way to work around this problem is to add multiSelect:true in the call.

So this is the proposal that has been made here, in the due_date_select.js file

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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 #201427 (closed)