Fix schema timezone handling for datePublished and dateModified

[meravi]

#23153.

Summary

• add a schema-specific date formatter that converts GMT post dates into the site timezone
• update schema Article and WebPage generators to use the site timezone for datePublished and dateModified
• add and update unit tests for the helper and schema generators

Testing

• composer test -- --filter Date_Helper_Test
• composer test -- --filter Article_Test
• composer test -- --filter WebPage_Test
• composer test -- --filter Schema_Generator_Test

Props ravikhadka