Change MR links to Issue links in towncrier changes

See discussion here: !522 (comment 2750454638)

The proposition is because of the following:

• Issues are user-created, and thus are less technical and typically references a specific problem
• Issues are generally having some discussion on the topic at hand, and sometimes has an intermediate work-around (via e.g. flags)
• Merge Requests are generally complex, more developer oriented and less useful for end-users.
• We don't need to re-commit after seeing the MR number.
• It forces us to open an issue for the thing we are working on.
• We keep the same number if we decided to close an MR but open a new one that's related.

(collected from the thread)