chore(deps): Update dependency bootswatch to v5

Author: renovate[bot]

src/homepage/html/include/base.html

@@ -8,7 +8,7 @@
     <title>{{ title }}</title>
     <link rel="icon" type="image/png" href="https://docs.spongepowered.org/{{ mainVersion }}/en/_static/favicon.ico">
 
-    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootswatch/3.3.7/flatly/bootstrap.min.css" integrity="sha256-r1WijW/SNMgOwk5LDk7QRHr6oVYYbYWMw/1kOXfYJfg=" crossorigin="anonymous">
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootswatch/5.2.1/flatly/bootstrap.min.css" integrity="sha512-wIYoddftNw7JnaeYbKG4yL4CGGBYBhmcFEw8qobAu8zsDmEW/ZxfeWXyNbiUZ4BE5me64k2JuJRP9r2/i0Ty1Q==" crossorigin="anonymous">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css" integrity="sha256-eZrrJcwDc/3uDhsdt61sL2oOBY362qM3lon1gyExkL0=" crossorigin="anonymous">
     <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Montserrat:600,700">
     <link rel="stylesheet" href="_static/css/spongedocs.css">