Fix build error with MSRV and relax MSRV to 1.73

[taiki-e]

a298081 set MSRV to 1.77, but this crate is not actually be able to built with 1.77.

$ cargo +1.77 build
   Compiling rustversion v1.0.17
   Compiling libc v0.2.158
   Compiling cfg-if v1.0.0
   Compiling log v0.4.22
   Compiling generator v0.8.3 (/Users/taiki/projects/forks/others/generator-rs)
error: the `cargo::` syntax for build script output instructions was added in Rust 1.77.0, but the minimum supported Rust version of `generator v0.8.3 (/Users/taiki/projects/forks/others/generator-rs)` is 1.77.
See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script for more information about build script outputs.

• First, this PR fixes the above error.
• Then, the code with the above fix applied is also compatible with 1.73 and later compilers, so this PR also relaxes MSRV to 1.73.

[Xudong-Huang]

Thanks!