fix: upgrade express from 4.17.1 to 4.18.1

Snyk has created this PR to upgrade express from 4.17.1 to 4.18.1.

See this package in npm:
https://www.npmjs.com/package/express

See this project in Snyk:
https://app.snyk.io/org/klemek/project/81af650f-8c8e-4116-b289-95415c85d8a7?utm_source=github&utm_medium=referral&page=upgrade-pr

package.json

@@ -7,7 +7,7 @@
     "@iarna/toml": "^2.2.3",
     "body-parser": "^1.19.0",
     "ejs": "^3.1.6",
-    "express": "^4.17.1",
+    "express": "^4.18.1",
     "express-rate-limit": "^5.0.0",
     "fa-diagrams": "^1.0.3",
     "mathjax-node": "^2.1.1",
@@ -16,7 +16,7 @@
     "prismjs": "^1.23.0",
     "redis": "^3.0.2",
     "rss": "^1.2.2",
-    "showdown": "^2.0.2"
+    "showdown": "^1.9.1"
   },
   "devDependencies": {
     "coveralls": "^3.0.4",