Fix a bug with promoting Issues with attachments to Epics

What does this MR do?

This MR fixes a bug when promoting an Issue with attachments to an Epic.

The bug was introduced in !38646 (merged) where the new MarkdownContentRewriterService was passed the target's #project (in two places: here and here), where previously new_parent was passed instead. new_parent was either the #project or the #group.

When an Issue is promoted to an Epic, the target was the Epic, and as Epic#project is nil, this meant that a nil was passed to MarkdownContentRewriterService as the target_parent, which would result in an error when that class called Gitlab::Gfm::UploadsRewriter and methods on nil were called (see #236190 (closed)).

This MR passed the object's #resource_parent to MarkdownContentRewriterService which will correctly be a Project or Group.

• Issue #236190 (closed)

QA Steps

1. Create a group, and a project within the group, and an issue within that project.
2. Attach an image to the issue, either in the issue's description, or in a comment on the issue.
3. Save a comment on the issue with the quick action: /promote

On master this will cause an error, on this branch it will promote the Issue to an Epic.

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