Decouple MSRV for `bindgen` and `bindgen-cli`

Last time we raised the MSRV was due to a `clap` update if I remember correctly. However, we don't need to do that for the `bindgen` crate because `clap` is a dependency for `bindgen-cli` only. If we kept two MSRV, it would allow us to use `bindgen` with a lower rust version than the `bindgen-cli` app (where having a higher MSRV is not as important as people just install it)