backups: Remove use of length_is template function

- It has been deprecated in Django 4.2

Tests:

- Add a remote backup location with valid SSH hostname and notice that the list
of SSH host keys are shown properly.

- Add a remote backup location with invalid SSH hostname and notice that the
error "Could not reach SSH host..." is shown.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/backups/templates/verify_ssh_hostkey.html

@@ -13,7 +13,7 @@
   <form class="form" method="post">
     {% csrf_token %}
 
-    {% if form.ssh_public_key|length_is:"0" %}
+    {% if form.ssh_public_key|length == 0 %}
       <p>
         {% blocktrans trimmed %}
           Could not reach SSH host {{ hostname }}. Please verify that the host