build(deps): bump lodash from 4.17.20 to 4.17.21 in /addon (#789)

Bumps [lodash](https://github.com/lodash/lodash) from 4.17.20 to 4.17.21.
- [Release notes](https://github.com/lodash/lodash/releases)
- [Commits](lodash/lodash@4.17.20...4.17.21)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

addon/package-lock.json

@@ -3,7 +3,7 @@
   "version": "0.11.0",
   "description": "Extension that augments several Mozilla websites (DXR, Searchfox) with code coverage information.",
   "dependencies": {
-    "lodash": "^4.17.20"
+    "lodash": "^4.17.21"
   },
   "peerDependencies": {
     "eslint": ">=0.8.0"