Add confidential parameter to epic link creation API

What does this MR do?

This allows specifying the confidentiality of an epic created using the epic links API as a child of another epic (i.e. POST groups/:id/epics/:epic_iid/epics). This is needed, because sub-epics of a confidential epic can only be confidential. Currently, trying to create a new sub-epic of a confidential epic using this endpoint fails (as described in #229839 (closed)). Consequently, the (reasonable) default for this new parameter is to copy the confidentiality status of the parent epic.

Additionally, this includes a minor frontend change to replicate this default behavior on the frontend. This is actual unnecessary, as omitting it has the same effect, but I felt it would be better to be independent from any backend defaults there. But I am happy to hear opinions on this.

Screenshots

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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 #229839 (closed)