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

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-BODYPARSER-7926860
- https://snyk.io/vuln/SNYK-JS-EXPRESS-7926867
- https://snyk.io/vuln/SNYK-JS-SEND-7926862
- https://snyk.io/vuln/SNYK-JS-SERVESTATIC-7926865

package.json

@@ -5,9 +5,9 @@
   "main": "src/server.js",
   "dependencies": {
     "@iarna/toml": "^2.2.3",
-    "body-parser": "^1.19.0",
+    "body-parser": "^1.20.3",
     "ejs": "^3.1.6",
-    "express": "^4.17.1",
+    "express": "^4.21.0",
     "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",