Updating PR template to add a checklist item (#171813)

**Related to:** #161505

## Summary

This PR adds a checklist item in the pull request template for this
repository. The added checklist item is to check if the [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on tests changed in the pull request.

### Checklist

- [X] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)

### For maintainers

- [x] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

.github/PULL_REQUEST_TEMPLATE.md

@@ -10,6 +10,7 @@ Delete any items that are not applicable to this PR.
 - [ ] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
 - [ ] [Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html) was added for features that require explanation or tutorials
 - [ ] [Unit or functional tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html) were updated or added to match the most common scenarios
+- [ ] [Flaky Test Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was used on any tests changed
 - [ ] Any UI touched in this PR is usable by keyboard only (learn more about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
 - [ ] Any UI touched in this PR does not create any new axe failures (run axe in browser: [FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/), [Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
 - [ ] If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the [docker list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)