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/3.4.0/flatly/bootstrap.min.css" integrity="sha512-eKDImTay+dzem1seYVlpQEXVUmjMXOd2SpnfIsqqkkCJd6pU0u4yGs5jSrFlTPwnugDdMdCm8WDrOiG8DXBwPg==" crossorigin="anonymous">
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootswatch/5.3.8/flatly/bootstrap.min.css" integrity="sha512-6cPfXE7P+s3gLoNVw8XfKNI3nc0kKcDIhZzlFp81ZLRcueIq2ayKiHcT9YT7Z+q0+61MKhvVaSaV5PmG7jEsKA==" 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">