Update to new interpolation syntax

Includes a fix to a brittle test that failed due to changes in the
number of linebreaks.

lib/music_library_web/components/layouts/unauthenticated.html.heex

@@ -1,6 +1,6 @@
 <main class="px-4 py-8 max-sm:pb-20 sm:px-6 lg:px-8">
   <div class="mx-auto max-w-5xl">
     <.flash_group flash={@flash} />
-    <%= @inner_content %>
+    {@inner_content}
   </div>
 </main>