PR Template: Suggest linking the issue (#68924)

Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: stokesman <presstoke@git.wordpress.org>
Co-authored-by: priethor <priethor@git.wordpress.org>
Co-authored-by: afercia <afercia@git.wordpress.org>

Author: 4 people

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,6 +2,9 @@
 https://github.com/WordPress/gutenberg/blob/trunk/CONTRIBUTING.md -->
 
 ## What?
+<!-- Link this PR to its associated issue with an appropriate keyword: Closes, See, Follow up to, etc. -->
+Closes <!-- #ISSUE-NUMBER or URL -->
+
 <!-- In a few words, what is the PR actually doing? -->
 
 ## Why?