update fd_rent_exempt_minimum_balance

[mubarizkyc]

Currently, fd_rent_exempt_minimum_balance follows this implementation. However, this should be updated to the new API here.
This change also involves updates to fd_rent_t, as the field name lamports_per_byte_year has been updated to lamports_per_byte, and the field exemption_threshold has been updated from f64 to [u8; 8] (source).
Happy to open a PR with these changes if the team considers it appropriate.
cc @ripatel-fd

[topointon-jump]

Thanks for this! Personally I would rather wait until deprecate_rent_exemption_threshold has been activated everywhere (few weeks) and then clean up the rent code, so that we don't churn this code twice.