Resolve "External tracker issues don't have the correct closing text appended to MRs"

What does this MR do?

Fix an issue with the closing issue text appended to an MR when using an external issue tracker.

Perhaps a table is easiest:

Tracker	Issue in branch name	Description before	Description after
JIRA	FOO-123	Closes FOO-123	Closes FOO-123
JIRA	None	Closes	Empty
External	1	Closes 1	Closes #1
External	None	Closes	Empty
Internal	1	Closes #1	Closes #1
Internal	None	Empty	Empty

Are there points in the code the reviewer needs to double check?

Does the above make sense? I changed the specs a lot to match that table.

Why was this MR needed?

It's a bug in 10.5.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #43240 (closed).