layouts: Fix a few outdated .font-italic uses

In Bootstrap v5 the font utility classes were renamed.

layouts/partials/sidebar.html

@@ -1,7 +1,7 @@
 <div class="col-md-4">
   {{ if and (.Site.Params.sidebar) (isset .Site.Params.sidebar "about") }}
 	<div class="p-4 mb-3 bg-light rounded">
-		<h4 class="font-italic">About</h4>
+		<h4 class="fst-italic">About</h4>
     <p class="mb-0">{{ .Site.Params.sidebar.about | markdownify }}</p>
 	</div>
   {{ end }}
@@ -12,7 +12,7 @@
 
   {{ with .Site.Menus.sidebar }}
 	<div class="p-4">
-		<h4 class="font-italic">Elsewhere</h4>
+		<h4 class="fst-italic">Elsewhere</h4>
 		<ol class="list-unstyled">
       {{ range . }}
       <li><a href="{{ .URL | absURL }}">{{ .Name }}</a></li>