Auto merge of #2713 - rust-lang:renovate/ember-source-3.x, r=Turbo87

Update dependency ember-source to v3.21.1

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [ember-source](https://emberjs.com/) ([source](https://togithub.com/emberjs/ember.js)) | devDependencies | minor | [`3.20.5` -> `3.21.1`](https://renovatebot.com/diffs/npm/ember-source/3.20.5/3.21.1) |

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### Release Notes

<details>
<summary>emberjs/ember.js</summary>

### [`v3.21.1`](https://togithub.com/emberjs/ember.js/blob/master/CHANGELOG.md#v3211-August-28-2020)

[Compare Source](https://togithub.com/emberjs/ember.js/compare/v3.21.0...v3.21.1)

-   [#&#8203;19106](https://togithub.com/emberjs/ember.js/pull/19106) [BUGFIX] Ensure `destroy` methods on `CoreObject` are invoked.
-   [#&#8203;19111](https://togithub.com/emberjs/ember.js/pull/19111) [BUGFIX] Fixes `ArrayProxy` length reactivity.

### [`v3.21.0`](https://togithub.com/emberjs/ember.js/blob/master/CHANGELOG.md#v3210-August-24-2020)

[Compare Source](https://togithub.com/emberjs/ember.js/compare/v3.20.5...v3.21.0)

-   [#&#8203;18993](https://togithub.com/emberjs/ember.js/pull/18993) [DEPRECATION] Deprecate `getWithDefault` per [RFC #&#8203;554](https://togithub.com/emberjs/rfcs/blob/master/text/0554-deprecate-getwithdefault.md).
-   [#&#8203;19087](https://togithub.com/emberjs/ember.js/pull/19087) [BUGFIX] Generated initializer tests no longer causes a deprecation warning
-   [#&#8203;17571](https://togithub.com/emberjs/ember.js/pull/17571) [BUGFIX] Avoid tampering `queryParam` argument in RouterService#isActive

</details>

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Author: bors

package-lock.json

@@ -89,7 +89,7 @@
     "ember-qunit": "4.6.0",
     "ember-resolver": "8.0.2",
     "ember-router-scroll": "2.0.2",
-    "ember-source": "3.20.5",
+    "ember-source": "3.21.1",
     "ember-svg-jar": "2.2.3",
     "ember-template-lint": "2.12.1",
     "ember-test-selectors": "5.0.0",