Fix(BreakoutRoomsEditor): Add amount validation

[DorraJaouad]

☑️ Resolves

• Fix Incorrect handling of invalid breakout rooms number #11355

Hint is conditioned because the user will likely pick an amount from 1 to 20, but it will be shown in case they attempt to choose outside that range.

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After

🚧 Tasks

• Code review
• Visual review

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖌️ 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

[Antreesy]

After checking the upstream code, it now make a little sense for me to literally upstream it 😬

We still could do at the place (with refs and without pure JS) but to expose checkValidity() from lib is not as needed as to expose focus(), for example.
In addition to that, NcInput is simple visual / low-functional component, so again, I don't see any point in making it more complicate upstream

[Antreesy]

/backport to stable28