feat: Support table headers in formatted text (DEV-3473)

[BalduinLandolt]

Pull Request Checklist

Task Description/Number

Issue Number: DEV-

PR Type

• build/chore: maintenance tasks (no production code change)
• docs: documentation changes (no production code change)
• feat: represents new features
• fix: represents bug fixes
• perf: performance improvements
• refactor: represents production code refactoring
• test: adding or refactoring tests (no production code change)
• deprecated: Deprecation warning (ideally referencing a migration guide)

Basic requirements for bug fixes and features

• Tests for the changes have been added
• Docs have been added / updated

Does this PR introduce a breaking change?

• Yes

Does this PR change client-test-data?

• Yes

[linear]

DEV-3473 CKEditor: using header row or merged cell in table leads to error

[seakayone]

question: As far as I understand the colspan tag is the problem in the bug report, isn't it?

or merged cell in table leads to error

[BalduinLandolt]

question: As far as I understand the colspan tag is the problem in the bug report, isn't it?

or merged cell in table leads to error

the merge cell feature has been deactivated in the frontend, so there was no immediate need to add that for now. We could re-enable it and add it too, but this was more immediately an issue, and I think it would make sense to ensure that this works first. But I'm open for discussion, if you think that should be done in one go