Changes 9: Remove content storage class

[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
• 🚨 Merge first: Changes 8: New ModelWithContent::version method #6455

Refactoring

• ModelWithContent and all it's children are now using the PlainTextContentStorage class without the wrapping ContentStorage class
• ContentStorage has been removed
• Language::create and Language::delete have been fixed

Breaking changes

• ContentStorage does no longer exist. Since it has been marked as internal, this should not be an issue.

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