Set MSRV to 1.70 (#279)

With the bindgen 0.70 upgrade, the default rust target is set to be 1.77,
which becomes the de facto MSRV of boring-sys since the change.

This change makes sure that the MSRV of boring-sys is kept at 1.70,
which is the same as that of bindgen.

boring-sys/build/main.rs

@@ -688,6 +688,7 @@ fn main() {
     });
 
     let mut builder = bindgen::Builder::default()
+        .rust_target(bindgen::RustTarget::Stable_1_68) // bindgen MSRV is 1.70, so this is enough
         .derive_copy(true)
         .derive_debug(true)
         .derive_default(true)