Drop multiple_security_releases FF

What does this MR do and why?

Removes multiple_security_releases from release-tools codebase. The feature flag has been enabled during 16.7 and 16.8 without any major issue.

Related to gitlab-com/gl-infra/delivery#19880 (closed)

Author Check-list

• [-] Has documentation been updated?

Testing

I executed the following to create the next security tracking issue:

14:45:00 ❯ TEST=true rake security:finalize:update_tracking_issue

The tracking issue considered the versions from two weeks from now (2024-02-13) and the associated release managers

2024-01-30 14:45:37.001449 I [dry-run] ReleaseTools::Security::TrackingIssue -- Fetching the active versions for 2024-02-13 14:45:37 -0600
2024-01-30 14:45:42.273304 I [dry-run] ReleaseTools::Security::TrackingIssue -- fetching release managers -- { :release_managers => [ "ggillies", "mayra-cabrera", "vglafirov" ] }

Note: I modified the code to print the above output

Rake output:

Click to expand

Security release: 16.8.x, 16.7.x, 16.6.x

⚠ Only Release Managers and members of the AppSec team can edit the description of this issue

---

Version issues:

16.8.x, 16.7.x, 16.6.x: {release task link}

Issues in GitLab Security

To include your issue and merge requests in this Security Release, please apply the security-target label to your security issues to have them evaluated and linked to this release tracking issue and included in this release.

⚠ If your security issues are not marked as related to this release tracking issue, their merge requests will not be included in the security release.

Branches to target in GitLab Security

Development cutoff: Master MRs should be approved and ready to merge by start of EMEA day on <2 days before the release date>

Your Security Implementation Issue should have 4 merge requests associated:

• One targeting the default branch and 3 backports
• Backports should target the stable branches for the versions mentioned included in this Security Release

Blog post

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