docs: update the discussion template so people won't create issues

[knqyf263]

Description

It is stated in the documentation that maintainers create GitHub issues and the community should not open, but people are unlikely to read it. To make it more straightforward, this PR adds a note to the Discussion template.

Related issues

• CycloneDX SBOM files generated by trivy are not compliant with corresponding CycloneDX schema #4900
• False positive CVE-2022-4065 reported against org.testng:testng (testng-7.7.1.jar) #4802
• Incorrect mapping of Applicable versions for CVE-2023-28858 & CVE-2023-28859 #4558

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).