Fix links to trainee maintainer issue templates

Why is this change being made?

The links to trainee backend/frontend/database engineer in the handbook are broken - they take me to the issue page, but do not select the expected Issue template. This MR fixes the link so that the Issue I'm directed to is populated with the expected template.

Relevant page:

• https://about.gitlab.com/handbook/engineering/workflow/code-review/

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what