Update due_dates.md

Try to fix lint error found in job [527325703](https://gitlab.com/free_zed/gitlab/-/jobs/527325703):
`doc/user/project/issues/due_dates.md:34 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2]`

doc/user/project/issues/due_dates.md

@@ -31,7 +31,6 @@ The last way to set a due date is by using _quick actions_, directly in an issue
 
 See [quick actions in documentation](../quick_actions.md) for more details.
 
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 ## Making use of due dates
 
 Issues that have a due date can be easily seen in the issue tracker,