Merge branch 'attach-existing-issue-toast-message' into 'master'

Show toast message when attaching vulnerabilities to existing issues

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/181429



Merged-by: Savas Vedova <svedova@gitlab.com>
Approved-by: David Pisek <dpisek@gitlab.com>
Approved-by: Savas Vedova <svedova@gitlab.com>
Reviewed-by: David Pisek <dpisek@gitlab.com>
Co-authored-by: Lorenz van Herwaarden <lvanherwaarden@gitlab.com>