Update GTM to capture trial form saasTrialGroup event

What does this MR do and why?

This MR resolves the first item in gitlab-com/marketing/digital-experience/buyer-experience#259 (closed).

It updates the trackSaasTrialGroup Google Tag Manager function to track saasTrialGroup events with the js-saas-trial-group selector, which allows us to track these events across https://gitlab.com/-/trials/select and https://gitlab.com/users/sign_up/groups/new

This is part of a broader effort to instrument GitLab with Google Tag Manager, tracked in gitlab-com/marketing/digital-experience&43 (closed).

How to set up and validate locally

1. Create a Google Tag Manager account and container ID. You should just need to follow the step 1 of the instructions. The GTM container can be empty - it just needs to exist.
2. Enable the relevant feature flags

   Feature.enable(:ecomm_instrumentation)
   Feature.enable(:gtm_nonce)
   Feature.enable(:gitlab_gtm_datalayer)

3. In gitlab.yml, add your GTM ID in the extra block for both the google_tag_manager_id and google_tag_manager_nonce_id values. Make sure to restart GDK (or do this step prior to starting GDK) for settings to take effect.

   extra:
    google_tag_manager_id: 'GTM-YOURIDHERE'
    google_tag_manager_nonce_id: 'GTM-YOURIDHERE'

4. Open the Google Tag Assistant and point it to http://localhost:3000/-/trial_registrations/new
5. Walk through the trial flow until you get to /-/trials/select
6. Create a group or use an existing group for your user
7. Click "start your free trial" and check that the Google Tag Assistant registers the saasTrialGroup in the dataLayer

I also added a feature spec and Jest spec to verify the behavior.

MR acceptance checklist

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