Add 'Untriaged' label to WPF GitHub PR template (#4135)

PULL_REQUEST_TEMPLATE

-- Please add description for changes you are making.
-- If there is an issue related to this PR, please add the reference.
+---
+labels: Untriaged
+
+---
+
+Fixes Issue <!-- Issue Number -->
+
+Master PR <!-- Link to PR if any that fixed this in the master branch. -->
+
+# Description
+
+<!-- Give a brief summary of the issue and how the pull request is fixing it. -->
+
+# Customer Impact
+
+<!-- What is the impact to customers of not taking this fix? -->
+
+# Regression
+
+<!-- Is this fixing a problem that was introduced in the most recent release, ie., fixing a regression? -->
+
+# Testing
+
+<!-- What kind of testing has been done with the fix. -->
+
+# Risk
+
+<!-- Please assess the risk of taking this fix. Provide details backing up your assessment. -->