Add MSRV to Cargo.toml

[ohAitch]

https://rust-lang.github.io/rfcs/2495-min-rust-version.html

I'm not sure how to determine said version, but apparently u64::MAX(vs std::u64::MAX) was added sometime after 1.41.1

[DJMcNab]

I don't think this is implemented in cargo yet (rust-lang/rust#65262), and it doesn't look like it's coming anytime soon.
I'd be in favour of closing this issue for the time being, as it isn't actionable from us.
I'm sure once rfc 2495 is implemented and stabilised, we would use it however.

[alice-i-cecile]

We've discussed this elsewhere several times; the churn of updating this isn't worth it right now. For the foreseeable future, Bevy will be on "latest stable" due to our ravenous appetite for new Rust fixes and features.