Update dependency mathjax to v4

Author: renovate[bot]

package.json

@@ -28,7 +28,7 @@
     "hexo-generator-searchdb": "1.5.0",
     "katex": "0.16.9",
     "lozad": "1.16.0",
-    "mathjax": "3.2.2",
+    "mathjax": "4.0.0",
     "medium-zoom": "1.1.0",
     "mermaid": "11.10.1",
     "pace-js": "1.2.4",