update prost to .6

[flipchan]

Hoy hoy,
Prost is unstable in versions less then 6 , (nasty stack overflow)

Link to more info:
https://rust.firosolutions.com/paste/Zar6yGo7tfohUUFqMDeFSFr0UexQ2b8mpXUGJ2uOY

Plz update your Cargo.toml file