Fix new project group templates pagination (16-10-stable-ee)

What does this MR do and why?

This is a backport of !147534 (merged) to 6-10-stable-ee

Implements a temporary fix for the new project group templates pagination as part of a priority1 bug fix.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).

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Before	After

How to set up and validate locally

Note: Testing requires an instance with many Group templates, the easiest for testing is 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..888f8bd8a4dc 100644
--- a/ee/app/controllers/ee/users_controller.rb
+++ b/ee/app/controllers/ee/users_controller.rb
@@ -34,6 +34,7 @@ def available_group_templates
       @groups_with_project_templates = # rubocop:disable Gitlab/ModuleWithInstanceVariables
         user.available_subgroups_with_custom_project_templates(params[:group_id])
           .page(params[:page])
+          .per(1)
           # Workaround: to generate correct COUNT sql:
           # https://gitlab.com/gitlab-org/gitlab/-/issues/381077
           .tap { |t| t.total_count("#{::Namespace.table_name}.#{::Namespace.primary_key}") }

1. Navigate to http://gdk.test:3000/flightjs and create a New subgroup
2. Navigate to the sub group and create a project under the subgroup
3. Navigate to http://gdk.test:3000/groups/flightjs/-/edit#js-custom-project-templates-settings
4. Under Custom project templates select the subgroup created in step 1
5. Follow step 1-4 for at least one more group.
6. Navigate to http://gdk.test:3000/projects/new#create_from_template
   • Replace http://gdk.test:3000 with your instance root URL.
7. Click on Group
8. The pagination on the Group tab should work as expected.

Related to #442350 (closed)