Revise GitlabInternalMailer to use mailer/new_mailer layout

Problem

Most emails use the mailer/new_mailer layout which are to be migrated, but the GitlabInternalMailer doesn't use this layout.

Discussion: #5567 (comment 1248914553)

Proposal

Migrate the GitlabInternalMailer to use the mailer/new_mailer layout.

Result

The email will have the same layout as the rest of the emails.