refactor: rename info() to header() (#6138)

This change renames all the `info()` functions to `header()`, since they return `LedgerHeader` structs. It also renames the underlying variables from `info_` to `header_`.

src/test/jtx/impl/Oracle.cpp

@@ -216,7 +216,7 @@ Oracle::set(UpdateArg const& arg)
     else
         jv[jss::LastUpdateTime] = to_string(
             duration_cast<seconds>(
-                env_.current()->info().closeTime.time_since_epoch())
+                env_.current()->header().closeTime.time_since_epoch())
                 .count() +
             epoch_offset.count());
     Json::Value dataSeries(Json::arrayValue);