Add GA4 tag to head

What does this MR do and why?

Describe in detail what your merge request does and why.

Closes gitlab-com/marketing/digital-experience/buyer-experience#922 (closed), which is pushing new consent events to the dataLayer for GitLab SaaS.

I looked at previous MRs of existing work and added the necessary script above the existing one within the same partial

Screenshots or screen recordings

https://youtu.be/q4A5LOSV7pc

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set GITLAB_SIMULATE_SAAS=1 in your environment to make GDK act as SaaS, since this change only affects SaaS.
2. 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.
3. In gitlab.yml, add a 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. Start or restart your GDK
5. bundle exec rails c to open the Rails console and enable some feature flags:
6. Turn on Feature.enable(:gitlab_gtm_datalayer)
7. Open the Google Tag Assistant
8. Visit http://localhost:3000/users/sign_up from the Tag Assistant
9. In the Left menu, check Consent and look for something like this
10. You can also skip the tag assistant and visit localhost:3000/users/sign_up and run window.dataLayer in the console and you should see a value like this:

[
    {
        "0": "consent",
        "1": "default",
        "2": {
            "analytics_storage": "denied",
            "ad_storage": "denied",
            "functionality_storage": "denied",
            "region": [
                "EU",
                "UK",
                "PE",
                "RU"
            ],
            "wait_for_update": 500
        }
    },
    {
        "0": "consent",
        "1": "default",
        "2": {
            "analytics_storage": "granted",
            "ad_storage": "granted",
            "functionality_storage": "granted",
            "wait_for_update": 500
        }
    },
    {
        "gtm.start": 1657736847101,
        "event": "gtm.js",
        "gtm.uniqueEventId": 3
    },
    {
        "event": "gtm.dom",
        "gtm.uniqueEventId": 4
    },
    {
        "event": "gtm.load",
        "gtm.uniqueEventId": 5
    }
]

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.