Enable trial onboarding flow for other sources

What does this MR do and why?

Ensure that the trial onboarding flow is enabled when a user is referred from docs.gitlab.com as well as learn.gitlab.com

part of: #349716

Changelog: changed

How to set up and validate locally

1. If you don't have Gitlab::SubscriptionPortal::Client configured properly, its probably easiest to do the following just to test the flow:

--- a/ee/app/controllers/trials_controller.rb
+++ b/ee/app/controllers/trials_controller.rb
@@ -32,7 +32,8 @@ def select

   def create_lead
     url_params = { glm_source: params[:glm_source], glm_content: params[:glm_content] }
-    @result = GitlabSubscriptions::CreateLeadService.new.execute({ trial_user: company_params })
+    #@result = GitlabSubscriptions::CreateLeadService.new.execute({ trial_user: company_params })
+    @result = { success: true }

2. Sign up via localhost:3000/-/trial_registrations/new?glm_source=learn.gitlab.com&glm_content=free-trial

3. After signing up, confirm you were redirected to http://localhost:3000/users/sign_up/groups/new?glm_content=free-trial&glm_source=learn.gitlab.com&trial_onboarding_flow=true

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.