use form_tag + submit_tag for SSO login button

Replaces button_to with form_tag + submit_tag so the generated
<input type="submit"> matches the standard ActiveAdmin theme CSS
selector (form input[type=submit]) without requiring custom styles.

Author: Fivell

app/views/active_admin/devise/sessions/new.html.erb

@@ -1,9 +1,7 @@
 <div id="login">
   <h2><%= active_admin_application.site_title(self) %></h2>
 
-  <%= button_to ActiveAdmin::Oidc.config.login_button_label,
-                "/admin/auth/oidc",
-                method: :post,
-                class: "activeadmin-oidc-login-button",
-                data: { turbo: false } %>
+  <%= form_tag "/admin/auth/oidc", method: :post, data: { turbo: false } do %>
+    <%= submit_tag ActiveAdmin::Oidc.config.login_button_label %>
+  <% end %>
 </div>