Downgrade MSRV to 1.51

[kangalio]

It would be nice to keep the MSRV low

• to have wider compatibility with Linux distros and Rust installs
• to not force breaking changes on dependent crates using tungstenite internally that have lower MSRV than tungstenite itself (like serenity)

[daniel-abramov]

Hm, ok, let's give it a try. I generally expected people to update the Rust version on a regular basis and the 1.51 is somewhat old 😉 But I can for sure understand the rationale of the change, so let's merge it then!