Update to jekyll 4 (modernization)

blog/index.html

@@ -11,7 +11,7 @@ layout: default
         <div class="col-md-12 post">
             <header class="post-header">
                 <h2 class="post-title">
-                    <a href="{{ post.url | prepend: site.baseurl }}">{{ post.title }}</a>
+                    <a href="{{ post.url | relative_url }}">{{ post.title }}</a>
                 </h2>
                 <p class="post-meta">
                     {{ post.date | date: "%b %-d, %Y" }}{% if post.author %} • {{ post.author }}{% endif %}{% if post.meta %} • {{ post.meta }}{% endif %}
@@ -23,7 +23,7 @@ layout: default
                 {% assign wordcount = post.content | number_of_words %}
                 {% if wordcount > 200 %}
                 <p>
-                    <a class="btn btn-default" href="{{ post.url | prepend: site.baseurl }}" role="button">&gt; Weiterlesen</a>
+                    <a class="btn btn-default" href="{{ post.url | relative_url }}" role="button">&gt; Weiterlesen</a>
                 </p>
                 {% endif %}
             </article>