Add MSRV policy to README and bump `rust_version` to 1.64, due to `ndk` -> `raw_window_handle` dependency

[rib]

This also updates the CI to test with cargo-ndk 2, which compiles with rustc 1.64 (whereas v3 requires 1.68)

[MarijnS95]

Title also has 0.64 but I think you meant 1.64 :)