Fix: Add hidden heading in _published_dates

[CodeSonia]

What

Trello card

Added a hidden heading Updates to this page to the metadata sections in the publishing_dates partial to properly indicate when the page was published and last updated for screen reader users.

Why

This fix addresses a DAC audit accessibility failure by ensuring screen readers can correctly identify and announce the metadata section.

Screenshots

Document Type	After	Link
Speech		https://government-frontend-pr-3262.herokuapp.com/government/speeches/what-makes-a-good-leader
Case study		https://government-frontend-pr-3262.herokuapp.com/government/case-studies/retirement-calculator-mccloud-legal-ruling
Fatality notice		https://government-frontend-pr-3262.herokuapp.com/government/fatalities/ministry-of-defence-confirms-the-death-of-squadron-leader-mark-long
News article		https://government-frontend-pr-3262.herokuapp.com/government/news/uk-launches-global-health-insurance-card
Statistical data set		https://government-frontend-pr-3262.herokuapp.com/government/statistical-data-sets/car-driving-test-data-by-test-centre
Specialist document type		https://government-frontend-pr-3262.herokuapp.com/tax-and-chancery-tribunal-decisions/elisabeth-moyne-ramsay-v-the-commissioners-for-hm-revenue-and-customs-2013-ukut-0226-tcc

[MartinJJones]

Nice work!, the changes look good to me, thanks for providing the links to the pages to check as well 👍