fix: package.json & package-lock.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-ANSIREGEX-1583908

package.json

@@ -7,7 +7,7 @@
     "@iarna/toml": "^2.2.3",
     "body-parser": "^1.19.0",
     "ejs": "^3.1.6",
-    "express": "^4.19.2",
+    "express": "^4.17.1",
     "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": "^1.9.1"
+    "showdown": "^2.0.2"
   },
   "devDependencies": {
     "coveralls": "^3.0.4",