Fix instance templates pagination (17-0-stable-ee)

What does this MR do and why?

This is a backport of !152921 (merged) to 17-0-stable-ee (see request for backport)

Fixes the broken UI when paginating instance templates on the new project (from template) page.

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

Note: Testing requires an instance with many Project templates, for testing it might be easier to force pagination with the following patch

diff --git a/ee/app/controllers/ee/users_controller.rb b/ee/app/controllers/ee/users_controller.rb
index 4caf45581ab4..a21cfeb44593 100644
--- a/ee/app/controllers/ee/users_controller.rb
+++ b/ee/app/controllers/ee/users_controller.rb
@@ -22,7 +22,7 @@ module UsersController
     # https://gitlab.com/gitlab-org/gitlab/-/issues/345897
     def available_project_templates
       @custom_project_templates = # rubocop:disable Gitlab/ModuleWithInstanceVariables
-        user.available_custom_project_templates(search: params[:search]).page(params[:page])
+        user.available_custom_project_templates(search: params[:search]).page(params[:page]).per(3)
 
       render layout: false
     end

1. Create a new group.
2. Navigate to the group and create many projects within the group.
3. Navigate to http://gdk.test:3000/admin/application_settings/templates.
4. Under Custom project templates select the group created in step 1, and click on Save changes.
5. Navigate to http://gdk.test:3000/projects/new#create_from_template.
6. Click on the tab labeled Instance.
7. The projects created in Step 2 should be listed.
8. The pagination on both the Instance and Group tab should work as expected.

Related to #456061 (closed)