Add maintenance notice

What does this MR do?

As part of Bump SAST Analyzer major version for 17.0 and r... (gitlab-org/gitlab#455489 - closed) • Craig Smith • 17.0, this MR adds a maintenance notice to the README file. The maintenance notice is based on the previously set ESLint notice.

What are the relevant issue numbers?

gitlab-org/gitlab#455489 (closed)

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 updated/added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer