Adjust logic for multiple planned releases

What does this MR do and why?

Release tools logic is updated to account for multiple planned releases in a single milestone. Particularly, the logic was updated on:

• The tracking issue class to use the targeted versions and the release managers associated with a specific date.
• The notify next release managers service executed on the due date of a security release, the service fetches the release managers assigned on the tracking issue, instead of the upcoming release managers.

The updates are under a feature flag.

Related to gitlab-com/gl-infra/delivery#19839 (closed) and gitlab-com/gl-infra/delivery#19840 (closed)

To do

• Add specs
• Refactor

Author Check-list

• [-] Has documentation been updated?