Bump ember-cli from 3.15.1 to 3.21.2

Bumps [ember-cli](https://github.com/ember-cli/ember-cli) from 3.15.1 to 3.21.2.
- [Release notes](https://github.com/ember-cli/ember-cli/releases)
- [Changelog](https://github.com/ember-cli/ember-cli/blob/master/CHANGELOG.md)
- [Commits](ember-cli/ember-cli@v3.15.1...v3.21.2)

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

Author: dependabot-preview[bot]

package.json

@@ -39,7 +39,7 @@
     "babel-eslint": "^10.0.3",
     "broccoli-asset-rev": "^3.0.0",
     "ember-auto-import": "^1.6.0",
-    "ember-cli": "~3.15.1",
+    "ember-cli": "~3.21.2",
     "ember-cli-dependency-checker": "^3.2.0",
     "ember-cli-inject-live-reload": "^2.0.1",
     "ember-cli-sri": "^2.1.1",