Limit the size of the summary field in the Security report

What does this MR do?

Truncates the vulnerabilities[].evidence.summary field to a maximum of 20,000 characters. If the summary is greater than this length, the first 20k characters remain and the text ...[truncated] is appended to the end.

This protects DAST and Rails against vulnerability findings that include large amounts of JavaScript.

What are the relevant issue numbers?

gitlab-org/gitlab#259394 (closed)

Related to gitlab-org/gitlab#326786 (closed) as the test introduced for that issue has very large summaries.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer