chore: update PR template to teach newcomers about issue referencing (#15255)

Added a line to close related issues in the PR template.

Author: teemingc

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,6 @@
-<!-- Your PR description here -->
+closes #<!-- Add the related issue number here. Repeat this line for each additional issue it closes -->
+
+<!-- Explain the goal of the PR, why it is needed, and what has been changed to achieve that goal -->
 
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