Set min Rust version to 1.51.0

[webmaster128]

Because of #685, we should only support Rust 1.51.0 for the final 0.14 release. Rust 1.51.0 is scheduled for release on March 25th.

[webmaster128]

1.51.0 will stabilize min_const_generics, which allows us to simplify a good bunch of cosmwasm-std code (see packages/std/src/binary.rs). We should definitely look into this update and require it if it helps.

[webmaster128]

Done in #852