Replacing basic-toml crate in dtolnay#239 raised MSRV
to 1.67 (toml_datetime specifies this minimum), but CI workflow was
updated to 1.70 in that PR, so this patch raises MSRV to 1.70
instead.

A reference to even older MSRV in the README is removed entirely
since this information lives in Cargo.toml as of Rust 1.56.

Upstream switched from basic-toml to toml in dtolnay/trybuild#239
which leads to a dependency on toml_datetime which has an MSRV of
Rust 1.67; since this is just a dev-dependency, there is no reason
to increase MSRV, so just pin the trybuild dependency to the
previous version until there is some new feature that requires
raising binrw MSRV again.