Improve Vue group select implementation

What does this MR do and why?

This improves the Vue group selector by:

• Moving the error handling to the component's template rather than relying on the createAlert helper.
• Enabling infinite scrolling. With this, we are achieving feature parity with the legacy select2 selector, which should let us remove the associated feature flag.

Screenshots or screen recordings

Error handling

Before	After

Infinite scroll

infscroll

How to set up and validate locally

1. Enable the vue_group_select feature flag.

echo "Feature.enable(:vue_group_select)" | rails c

1. Navigate to one of the affected pages:
   • Admin Area > Settings > Templates > Custom project templates (/admin/application_settings/templates#js-custom-project-templates-settings).
   • :group > Settings > General > Custom project templates (/groups/:group/-/edit).

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Related to #382717 (closed)