feat(mentions): prevent mention `@all` for non moderators

[sanskar-soni-9]

☑️ Resolves

• Fix #9074

🛠️ API Checklist

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🏁 Checklist

• [ ] ⛑️ Tests (unit and/or integration) are included or not possible
• [ ] 📘 API documentation in docs/ has been updated or is not required
• [ ] 🔖 Capability is added or not needed

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🖌️ UI Checklist

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🏁 Checklist

• [ ] 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• [ ] 🖥️ Tested with Desktop client or should not be risky for it
• [ ] 🖌️ Design was reviewed, approved or inspired by the design team
• [ ] ⛑️ Tests are included or not possible
• [ ] 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required