Bump MSRV from 1.61.0 to 1.70.0

Found using `cargo msrv find`, the `1.70.0` version is determined by `half=2.4.1` having an MSRV of 1.70.0.

Author: weiji14

Cargo.toml

@@ -5,7 +5,7 @@ edition = "2021"
 resolver = "2"
 
 # note: when changed, also update test runner in `.github/workflows/rust.yml`
-rust-version = "1.61.0"
+rust-version = "1.70.0"
 
 license = "MIT"
 description = "TIFF decoding and encoding library in pure Rust"