Fix slack notification for version tagging

What does this MR do and why?

The tagged version is with the prefix v, so the link the the slack message should also reflex that.

For example:

• Link
  • Before change: https://gitlab.com/gitlab-org/security/gitlab-foss/-/tags/16.5.4
  • After change : https://gitlab.com/gitlab-org/security/gitlab-foss/-/tags/v16.5.4
• Slack message:
  • Before change: GitLab Release Tools Bot tagged 16.5.4 on gitlab-org/security/gitlab-foss as a security release
  • After change : GitLab Release Tools Bot tagged v16.5.4 on gitlab-org/security/gitlab-foss as a security release

Content

• Fix slack notification for version tagging

Author Check-list

• Has documentation been updated?