Use font awesome lock icon that is available under free license

Author: minalike

pydis_site/apps/resources/templatetags/get_category_icon.py

@@ -21,7 +21,7 @@
     "Paid": "fa-dollar-sign",
     "Podcast": "fa-microphone-alt",
     "Project Ideas": "fa-lightbulb-o",
-    "Security": "lock-alt",
+    "Security": "fa-solid fa-lock",
     "Software Design": "fa-paint-brush",
     "Subscription": "fa-credit-card",
     "Testing": "fa-vial",