Upgrade react-markdown

[coder13]

Summary

• upgrades the client from react-markdown 4.3.1 to 6.0.3
• updates lobby announcements to use the v6 children API instead of the deprecated source prop
• adds a focused announcement markdown render test
• removes the old remark-parse 5 / trim 0.0.1 dependency path from the lockfile

Validation

• yarn workspace letscube-client test --watchAll=false
• yarn lint
• yarn build
• yarn test
• yarn cypress:run