Resolve "Project#to_reference does not produce a valid reference"

What does this MR do?

This adds a new method: Referable#to_reference_base to be used when references have child references with different formats from their own full format.

This is important for objects such as projects where their reference format is not marked by a prefix but by a post-fix.

This ensures that for all Referable objects, referable.to_reference produces a valid reference that can be parsed by the appropriate reference filter.

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

Closes #198116 (closed)