Changes 16: ModelWithContent simplifications

[bastianallgeier]

This PR …

• [x] 🚨 Merge first: #6436
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Reasoning

I'm trying to already use the improvements and changes from the last PRs in ModelWithContent without taking too many steps at once. The full conversion will be still a lot of complicated, potentially breaking steps. Taking it easy will help to keep our mental load in check.

Refactoring

• Simplify ModelWithContent::writeContent by using the new Version::save() method.
• Simplify ModelWithContent::content by using Language::ensure() and Version::content()

Breaking changes

None expected

Ready?

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

For review team

• [x] Add changes & docs to release notes draft in Notion