Bump lodash from 4.17.4 to 4.17.21 in /generator-exercise

Bumps [lodash](https://github.com/lodash/lodash) from 4.17.4 to 4.17.21.
- [Release notes](https://github.com/lodash/lodash/releases)
- [Commits](https://github.com/lodash/lodash/compare/4.17.4...4.17.21)

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

generator-exercise/package-lock.json

@@ -3098,9 +3098,9 @@
       }
     },
     "lodash": {
-      "version": "4.17.4",
-      "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.17.4.tgz",
-      "integrity": "sha1-eCA6TRwyiuHYbcpkYONptX9AVa4="
+      "version": "4.17.21",
+      "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.17.21.tgz",
+      "integrity": "sha512-v2kDEe57lecTulaDIuNTPy3Ry4gLGJ6Z1O3vE1krgXZNrsQ+LFTGHVxVjcXPs17LhbZVGedAJv8XZ1tvj5FvSg=="
     },
     "log-symbols": {
       "version": "1.0.2",