Update on SalesForce if a user opts to receive updates about GitLab

What does this MR do?

As per the requirement, the attribute newsletterSegment__c needs to sent to SalesForce as true, if a user opts to receive updates via email about GitLab, while signing up for trial.

Lead creation for trials is done in customers app.

This MR adds the following changes:

1. Sends the param newletter_segment to the Trials create api in customer's portal, in case the user opts in for trials.
2. Removes the system hook trigger to send Marketo event on trial user creation (refer to issue description for more info on the reason).
3. UI: Adds fix to send email_opt_in parameter in the form.

Customer app MR: https://gitlab.com/gitlab-org/customers-gitlab-com/merge_requests/663

Mentions #35459 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team