Backport of 'Fix Start free trial link for self-managed instances'

What does this MR do and why?

The new_trial_url helper method in ee/app/views/dashboard/projects/_blank_state_ee_trial.html.haml incorrectly generates a link to the local instance's /-/trials/new route on Self-Managed and GDK installations. This route only exists on GitLab.com, causing a 404 error on all other instances. The link should instead point to the GitLab Customer Portal for Self-Managed trial registration.

Backporting !205380 (merged) We usually backport bugs only to 1 release back. But there is an ongoing internal pilot is allowing GitLab engineers to backport bug fixes into all the maintained versions

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/570662

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.