Remove issue_date_filter feature flag and enable date filtering

What does this MR do and why?

Follow-up to !234283 (closed), which was closed due to inactivity. Removes the issue_date_filter feature flag and enables date filtering (created date, closed date, due date, updated date) for all users on the issue list.

The flag was blocked on the accessibility issue #420266 (keyboard navigation for the date token in the filtered search bar). That was fixed and merged in !239696 (merged), so this can now proceed.

This MR:

• Cherry-picks the original change from !234283 (closed) (hardcodes the flag check to true in the helper and GraphQL type, deletes the flag definition, removes flag-based tests).
• Fixes ee/spec/requests/api/graphql/namespaces/licensed_features_spec.rb, which wasn't touched by the original MR and broke against current master because it still stubbed the flag to false.
• Documents the date filters as generally available in doc/user/project/issues/managing_issues.md.

References

• Relates to #17758 (doesn't close it, relative dates are not implemented yet)
• Original MR: !234283 (closed)
• Accessibility fix that unblocked this: !239696 (merged)

How to set up and validate locally

1. Visit any issues list page, for example http://127.0.0.1:3000/dashboard/issues.
2. Confirm the date filter tokens (Created date, Closed date, Due date, Updated) are visible in the filtered search bar without enabling any feature flag.