meta: ask expected behavior reason in bug template

It happens too often that bug reporters only report _what_ they expect
to see without motivating _why_. Having to repeatedly ask for that info
gets old.

PR-URL: #47049
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Debadree Chatterjee <debadree333@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: bnoordhuis

.github/ISSUE_TEMPLATE/1-bug-report.yml

@@ -33,7 +33,7 @@ body:
       label: How often does it reproduce? Is there a required condition?
   - type: textarea
     attributes:
-      label: What is the expected behavior?
+      label: What is the expected behavior? Why is that the expected behavior?
       description: If possible please provide textual output instead of screenshots.
   - type: textarea
     attributes: