Deprecate unlimited children for epics to enforce a total limit of 100 children per epic

Summary

This issue tracks the deprecation of the ability to have unlimited child issues and epics on an epic and instead enforces a total limit of 100 children (i.e. issues & epics combined). We're doing this with following rationale;

• Epics are meant to be used for defined initiatives or features, something specific you plan to deliver.
• If there are over 100 issues around such an initiative, they should be categorised further via child epics.
• Theoretically, a single Epic can have 10,000 children in total even with this enforced limit by having 100 children per child epic.

Towards this deprecation, we need to formally announce it on UI (under Child issues and epics component) as well as mention it in https://docs.gitlab.com/ee/update/deprecations

📣 Roll-out plan

• We will announce a deprecation, introducing the new epic direct child limit of 100 in 17.1 or 17.2 if we miss the cutoff.
• We will raise the limit on work items so that the current experience is not disrupted when we move to the new epic experience.
• We will implement the new 100 direct child limit on epics in 18.0 and update the deprecation status to removal.
• Issue #463814 (closed) has been created for additional discussion of the enforcement which will come in 18.0.