Update to jekyll 4 (modernization)

index.html

@@ -9,7 +9,7 @@ show_mobile_large_logo: true
     {% for post in posts limit:3 %}
     <div class="col-md-4">
       <h2>
-          <a href="{{ post.url | prepend: site.baseurl }}">
+          <a href="{{ post.url | relative_url }}">
               {{ post.title }}
           </a>
       </h2>
@@ -19,7 +19,7 @@ show_mobile_large_logo: true
       {% assign wordcount = post.content | number_of_words %}
       {% if wordcount > 30 %}
         <p>
-            <a class="btn btn-default" href="{{ post.url | prepend: site.baseurl }}" role="button">
+            <a class="btn btn-default" href="{{ post.url | relative_url }}" role="button">
                 &gt; Weiterlesen
             </a>
         </p>