replace toolchain.toml with rust-version in Cargo.toml

[Sherlock-Holo]

rust-lang/cargo#8072 has supported rust-version in Cargo.toml, replace the toolchain.toml should avoid downloading old version toolchain when user have a new version toolchain, but also limit the newer api usage

[Sherlock-Holo]

and the current stable version

rustc 1.65.0 (897e37553 2022-11-02)

can build the PR #195, however the toolchain.toml version report failed

error[E0658]: trait bounds other than Sized on const fn parameters are unstable
--> /home/sherlock/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/enum-iterator-1.1.3/src/lib.rs:82:26
|
82 | pub const fn cardinality<T: Sequence>() -> usize {
| ^
|
= note: see issue #93706 rust-lang/rust#93706 for more information

For more information about this error, try rustc --explain E0658.
error: could not compile enum-iterator due to previous error
warning: build failed, waiting for other jobs to finish...
error: build failed

maybe git-trim can update the rust toolchain version to latest?