Allow a Release's release date (`released_at` attribute) to be added through the "Create Release" page

Problem to solve

Adding a Release's release date is only possible through the Releases API.

Proposal

Add a new field to the "Create Release" page that allows a user to set the release date (released_at api attribute).

User story

As a Release Manager, I want to add a release date to my release item using the application's UI, so that I can manage when the release will be/was ready.

🖼 SEE UX PROTOTYPES IN THE DESIGN TAB 🖼

NOTE: Prototypes are linked to #39471 (closed)

Acceptance criteria

• On the Release form, user sees a new section called Release date. This section should display a helper text and link to the documentation, as well as an input date.
  • The data added to the field should populate the released_at attribute for the release api.
• A date can be set in the past, present, or future.
• The field is not mandatory, and defaults to the current date.
• User should be able to enter a date to the release manually, by typing into the field.
• Clicking calendar icon present in the input date field triggers a dropdown with a datepicker. Guidelines from Pajamas should apply.
  • Selecting a date from the calendar should populate the input date field.
• If the date selected is in the future, the release item will be labeled an Upcoming Release.
  • Once a future date is inputted/selected, a helper text should be displayed under the input field indicating an Upcoming release will be created.
• Clicking the button to Save changes will update the released at or released in information on the Release item page and overview.

Further details

This feature should will also be available when editing a Release through the UI.

Documentation

Yes, we will need to update the documentation. Will likely need to be updated with new screenshots and instructions.