fix(MessagesSystemGroup): Follow-up correct collapsed property

[DorraJaouad]

☑️ Resolves

• regression from fix(MessagesSystemGroup): collapse actor removed messages, collapse previously open groups with unread marker #11876

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

Both screenshots have last read message is the last system message shown

🏚️ Before	🏡 After

🚧 Tasks

TO-DO next:

• MessagesSystemGroup component should contain only one group (1 combined message and its collapsed messages)
• Move groupIsCollapsed logic to MessagesList, so it can maintain manual collapsed state when switching between conversations.

🏁 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

[Antreesy]

Tested

[DorraJaouad]

/backport to stable29