Reformat email sent for exported vulnerability and dependency reports

The email sent when creating an export from the Vulnerability and Dependency pages can be confusing. We should make it more apparent what link will access the generated download.

Current email:

Changes needed

1. Move the "Download the export..." to a new line and bold if possible.
2. Split the email template and revise the text to "The security dashboard PDF report was successfully created for..."

Files

1. https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/views/vulnerabilities/export_mailer/completion_email.html.haml