Add default MR template for security products

Problem to solve

Our security products do not have merge request templates. Adding templates will provide us a checklist of best practices for both submitters and reviewers to align. This should improve both consistency and code quality.

Target audience

GitLabbers and contributors to ~Secure products

Proposal

Add a Merge Request description template to the following projects:

• https://gitlab.com/gitlab-org/security-products/analyzers/**/*
• https://gitlab.com/gitlab-org/security-products/sast
• https://gitlab.com/gitlab-org/security-products/dast
• https://gitlab.com/gitlab-org/security-products/license-management
• https://gitlab.com/gitlab-org/security-products/dependency-scanning
• https://gitlab.com/gitlab-org/security-products/codequality
• https://gitlab.com/gitlab-org/security-products/clair-scanner

What does success look like, and how can we measure that?

• MRs have consistent descriptions, labels, metadata
• All changes to semver'd products include CHANGELOG entries
• Review process is well-documented ok/product/pricing/#four-tiers )

Links / references

cc @gl-secure