Add the datePicker feature flag. #13609

Feature Description

Implement the datePicker feature flag to control the availability of the dashboard date picker during development and rollout.

While the flag is off, the existing date range dropdown remains the live path in the dashboard header.

For reference, see the Feature flag section in the Design Doc.

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• A new feature flag, datePicker, is added to the list of available feature flags.
• It's possible to toggle this feature on/off using the Tester plugin.

Implementation Brief

• Add the datePicker feature flag to the list of feature flags in feature-flags.json.

Test Coverage

• No new tests needed.

QA Brief

• Ensure datePicker feature flag is available in the tester plugin.
• It can be toggled.

Changelog entry

• Add the datePicker feature flag for the dashboard date picker feature.