feat(examples) Update C++ example

[charlesbvll]

Issue

Description

Related issues/PRs

Proposal

Explanation

Checklist

• Implement proposed change
• Write tests
• Update documentation
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?

[jafermarq]

@charlesbvll, some additional changes are needed to make it compatible with flwr>=1.11.0, the main one being that flower-server-app does not accept a reference to a ServerApp as input argument. Instead, it needs to be read from a pyproject.toml. Also, in the README.md, flwr-superlink should be replaced with flower-superlink.

[jafermarq]

@charlesbvll , the example looks ready to me. But we might need to merge first the changes in src/cc/flwr first in a separate PR...