chore(deps-dev): bump ember-cli-flash from 3.0.0 to 6.0.0 in /ui

Bumps [ember-cli-flash](https://github.com/adopted-ember-addons/ember-cli-flash) from 3.0.0 to 6.0.0.
- [Release notes](https://github.com/adopted-ember-addons/ember-cli-flash/releases)
- [Changelog](https://github.com/adopted-ember-addons/ember-cli-flash/blob/master/CHANGELOG.md)
- [Commits](https://github.com/adopted-ember-addons/ember-cli-flash/commits)

---
updated-dependencies:
- dependency-name: ember-cli-flash
  dependency-version: 6.0.0
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

ui/package.json

@@ -82,7 +82,7 @@
     "ember-cli-clipboard": "^1.0.0",
     "ember-cli-dependency-checker": "^3.2.0",
     "ember-cli-deprecation-workflow": "^2.1.0",
-    "ember-cli-flash": "^3.0.0",
+    "ember-cli-flash": "^6.0.0",
     "ember-cli-funnel": "^0.6.1",
     "ember-cli-htmlbars": "^5.7.2",
     "ember-cli-inject-live-reload": "^2.1.0",