Changes 8: New ModelWithContent::version method

[bastianallgeier]

This PR …

• 🚨 Merge first: Changes 1: Refactor content storage to use new VersionId class #6436
• 🚨 Merge first: Changes 2: Plain Text Handler Refactoring #6439
• 🚨 Merge first: Changes 3: New Version class #6442
• 🚨 Merge first: Changes 4: Refactor PlainTextContentStorageHandler to use Language object #6448
• 🚨 Merge first: Changes 5: ContentStorageHandler Abstract #6449
• 🚨 Merge first: Changes 6: Version class improvements and unit tests #6450
• 🚨 Merge first: Changes 7: Move ContentStorage methods to ContentStorageHandler #6454

Features

• New ModelWithContent::version() method

Refactoring

• Use new version method wherever it makes sense.
  • ModelWithContent::convertTo
  • ModelWithContent::readContent
  • ModelWithContent::writeContent
  • File::modifiedContent
  • Page::modified
  • System::exposedFileUrl
  • User::exists
  • User::modified
  • ContentTranslation::contentFile
  • FileActionsTest
  • PageActionsTest

The other methods are hard to test for the abstract class. We should probably add tests for the PlainTextContentStorageHandler class but so far all those tests have been missing anyway.

Breaking changes

• ModelWithContent::version() is now a reserved keyword. If you've used a field with this name, you need to use $model->content()->get('version') instead to work with the field.
• Page::modified(), File::modifiedContent() and User::modified() now use the current language instead of the default language.

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and checks all pass

For review team

• Add changes & docs to release notes draft in Notion