Move iteration cadences entry points to Vue 3 rollout

What does this MR do and why?

This MR opts the iteration cadences pages into the Vue 3 rollout.

It adds a vue3_migration.yml entry point configuration for the groups and projects iteration cadences pages, so those bundles run Vue 3 in compatibility mode once the new vue3_migrate_iteration_cadences beta feature flag is enabled. No behaviour changes are expected, since the pages should render and work exactly the same under both Vue versions.

References

Feature issue: #608108 (closed) Rollout issue: #612741

Screenshots

Group iteration	Project iteration

How to set up and validate locally

1. In the rails console, enable the feature flag:

   Feature.enable(:vue3_migrate_iteration_cadences)

2. Visit a group's iteration cadences page at /:group/-/cadences and confirm it loads correctly.

3. Visit a project's iteration cadences page and confirm it loads correctly.

4. Confirm that the browser console stays free of Vue errors on both pages.

MR acceptance checklist

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Related to #608108 (closed)