Changes 4: Refactor PlainTextContentStorageHandler to use Language object

[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

Refactoring

• New Language::single() method to create a Language placeholder object in single language installations
• Use full language objects in ContentStorageHandler and PlainTextContentStorageHandler methods
• Refactor ContentStorage to always provide a full language object to the storage methods. This is only temporary to keep all the unit tests working. ContentStorage will be removed later, as planned.

Outlook

• The PlainTextContentStorageHandler class will no longer implement the interface but extend the abstract ContentStorageHandler class. This change will also remove the __construct method, which will then be implemented by the abstract. Changes 5: ContentStorageHandler Abstract #6449
• The new ::write method will later be moved to the ContentStorageHandler class as an abstract method. ContentStorageHandler will then also get a non-abstract update and create method, which will use the new write method by default. This will add the option for storage handlers to only modify the write method if no further logic is needed for create and update. Changes 10: New MemoryContentStorageHandler #6457

Breaking changes

None

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

[lukasbestle]

That all looks very sensible now. 👍
The ContentStorage class is in a weird limbo now, but as it will get replaced anyway quite soon, it doesn't really matter.