This PR renames `LedgerInfo` to `LedgerHeader`. Namely, `LedgerInfo` was already an alias for `LedgerHeader`, and the comments next to the alias suggested that it would make sense to rename it, since that makes it clearer what it is.
Per XLS-0095, we are taking steps to rename ripple(d) to xrpl(d).
This change specifically removes all copyright notices referencing Ripple, XRPLF, and certain affiliated contributors upon mutual agreement, so the notice in the LICENSE.md file applies throughout. Copyright notices referencing external contributions remain as-is. Duplicate verbiage is also removed.
