Update and rename flaky-test.md to flaky-test.yml (#4099)

Otherwise the form is not rendered.

Author: rdner

.github/ISSUE_TEMPLATE/flaky-test.md .github/ISSUE_TEMPLATE/flaky-test.yml

@@ -1,4 +1,3 @@
----
 name: "Flaky Test"
 about: "Report a flaky test (one that doesn't pass consistently)"
 title: "[Flaky Test]: <test_case> – <short_error_message>"
@@ -52,4 +51,3 @@ body:
     render: markdown
     validations:
       required: true
----