Trim database changes MR template and require up-to-date description before sending an MR for review

What does this MR do?

Updates the Database changes MR template. Emphasizes adding a detailed description and keeping it up-to-date in response to discussion.

It also trims some items from the checklist that aren't always necessary and that can be automatically added when they are needed. These changes match changes made to the default MR template as discussed in #49692 (moved). The relatively new code owners features mean if review by a database specialist is required they can be suggested as approvers (https://gitlab.com/gitlab-org/gitlab-ee/issues/5382) or (soon) automatically included (https://gitlab.com/gitlab-org/gitlab-ee/issues/1012)

What are the relevant issue numbers?

#49692 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)