settings: Add a new `topics_policy` realm setting.

[Vector73]

Added a new realm setting, topics_policy and deprecated mandatory_topics setting as it now replaced by topics_policy. Past thread: #34150

Fixes #33549.

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Self-review checklist

• [x] Self-reviewed the changes for clarity and maintainability (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• [ ] Explains differences from previous plans (e.g., issue description).
• [ ] Highlights technical choices and bugs encountered.
• [ ] Calls out remaining decisions and concerns.
• [ ] Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• [x] Each commit is a coherent idea.
• [x] Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• [x] Visual appearance of the changes.
• [ ] Responsiveness and internationalization.
• [ ] Strings and tooltips.
• [ ] End-to-end functionality of buttons, interactions and flows.
• [ ] Corner cases, error conditions, and easily imagined bugs.