Set MSRV to 1.70 (#25)

Determined using `cargo msrv find --write-msrv`. See also https://gribnau.dev/cargo-msrv/releases/v0.15_v0.16_highlights.html#cargo-msrv-find---write-msrv

Author: weiji14

Cargo.toml

@@ -8,6 +8,7 @@ authors = [
     "Gunnar Schulze <[email protected]>"
 ]
 repository = "https://github.com/georust/geotiff"
+rust-version = "1.70"
 
 [dependencies]
 delaunator = { version = "1.0", optional = true }