build(deps): [security] bump lodash from 4.17.15 to 4.17.21 in /site

Bumps [lodash](https://github.com/lodash/lodash) from 4.17.15 to 4.17.21. **This update includes security fixes.**
- [Release notes](https://github.com/lodash/lodash/releases)
- [Commits](https://github.com/lodash/lodash/compare/4.17.15...4.17.21)

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

site/package.json

@@ -21,7 +21,7 @@
     "hexo-renderer-sass": "^0.4.0",
     "hexo-renderer-stylus": "^1.1.0",
     "hexo-server": "^1.0.0",
-    "lodash": "^4.17.13",
+    "lodash": "^4.17.21",
     "lunr": "^2.3.8",
     "uuid": "^7.0.2"
   },

site/yarn.lock

@@ -3514,10 +3514,10 @@ lodash@^3.6.0:
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-3.10.1.tgz#5bf45e8e49ba4189e17d482789dfd15bd140b7b6"
   integrity sha1-W/Rejkm6QYnhfUgnid/RW9FAt7Y=
 
-lodash@^4.0.0, lodash@^4.17.10, lodash@^4.17.11, lodash@^4.17.13, lodash@^4.17.14, lodash@^4.17.15, lodash@^4.17.4, lodash@~4.17.10:
-  version "4.17.15"
-  resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.15.tgz#b447f6670a0455bbfeedd11392eff330ea097548"
-  integrity sha512-8xOcRHvCjnocdS5cpwXQXVzmmh5e5+saE2QGoeQmbKmRS6J3VQppPOIt0MnmE+4xlZoumy0GPG0D0MVIQbNA1A==
+lodash@^4.0.0, lodash@^4.17.10, lodash@^4.17.11, lodash@^4.17.14, lodash@^4.17.15, lodash@^4.17.21, lodash@^4.17.4, lodash@~4.17.10:
+  version "4.17.21"
+  resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.21.tgz#679591c564c3bffaae8454cf0b3df370c3d6911c"
+  integrity sha512-v2kDEe57lecTulaDIuNTPy3Ry4gLGJ6Z1O3vE1krgXZNrsQ+LFTGHVxVjcXPs17LhbZVGedAJv8XZ1tvj5FvSg==
 
 longest@^1.0.1:
   version "1.0.1"