Replace YAML with TOML

[mmatyas]

The server config is currently stored in YAML, which is an unnecessarily complex format for our minimal needs.

• Get familiar with https://github.com/ToruNiina/toml11, add it using CMake FetchContent
• Compare its complexity and build size with the current YAML dependency
• Investigate whether reading and writing would be simpler using TOML
• If everything looks good, replace YAML with TOML (don't worry about format migration)