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/controllers/session_html/new.html.heex

@@ -2,13 +2,13 @@
   <div class="w-full max-w-sm space-y-10">
     <div>
       <h2 class="mt-10 text-center text-2xl font-bold leading-9 tracking-tight text-zinc-900 dark:text-zinc-200">
-        <%= gettext("Welcome to your Music Library") %>
+        {gettext("Welcome to your Music Library")}
       </h2>
     </div>
     <.form class="space-y-6" for={@form} action={~p"/sessions/create"}>
       <div class="relative -space-y-px rounded-md shadow-sm">
         <div>
-          <label for="password" class="sr-only"><%= gettext("Password") %></label>
+          <label for="password" class="sr-only">{gettext("Password")}</label>
           <input
             id="password"
             name="password"
@@ -41,7 +41,7 @@
             "focus-visible:outline focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-zinc-600"
           ]}
         >
-          <%= gettext("Login") %>
+          {gettext("Login")}
         </button>
       </div>
     </.form>