Migrate trial's namespace selector away from select2

What does this MR do and why?

This migrates the trial namespace selector away from select2 in favor of GlListbox.

Screenshots or screen recordings

	Before	After
Nothing selected
Create group selected
Existing group selected
Dropdown open

How to set up and validate locally

1. Apply the following patch to ensure you can access the affected page.

   diff --git a/ee/app/controllers/trials_controller.rb b/ee/app/controllers/trials_controller.rb
   index 46770eb524a..51d5144f660 100644
   --- a/ee/app/controllers/trials_controller.rb
   +++ b/ee/app/controllers/trials_controller.rb
   @@ -10,12 +10,12 @@ class TrialsController < ApplicationController
    
      layout 'minimal'
    
   -  before_action :check_if_gl_com_or_dev
   -  before_action :authenticate_user!, except: [:create_hand_raise_lead]
   -  before_action :authenticate_user_404!, only: [:create_hand_raise_lead]
   -  before_action :find_or_create_namespace, only: :apply
   -  before_action :find_namespace, only: [:extend_reactivate, :create_hand_raise_lead]
   -  before_action :authenticate_namespace_owner!, only: [:extend_reactivate]
   +  # before_action :check_if_gl_com_or_dev
   +  # before_action :authenticate_user!, except: [:create_hand_raise_lead]
   +  # before_action :authenticate_user_404!, only: [:create_hand_raise_lead]
   +  # before_action :find_or_create_namespace, only: :apply
   +  # before_action :find_namespace, only: [:extend_reactivate, :create_hand_raise_lead]
   +  # before_action :authenticate_namespace_owner!, only: [:extend_reactivate]
      before_action only: [:new, :select] do
        push_frontend_feature_flag(:gitlab_gtm_datalayer, type: :ops)
      end

2. Navigate to http://gdk.test:3000/-/trials/select

MR acceptance checklist

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

Related to #374095 (closed)