fix: unpin nginx

src/charmed_server.py

@@ -54,7 +54,7 @@ http {{
 def install() -> None:
     """Install nginx apt package."""
     apt.update()
-    apt.add_package("nginx", "1.18.0-6ubuntu14.8")
+    apt.add_package("nginx")
     subprocess.check_output(["service", "nginx", "stop"])
     os.remove("/etc/nginx/nginx.conf")
     os.unlink("/etc/nginx/sites-enabled/default")