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/app/Offer_test.cpp

@@ -20,7 +20,10 @@ class OfferBaseUtil_test : public beast::unit_test::suite
     std::uint32_t
     lastClose(jtx::Env& env)
     {
-        return env.current()->info().parentCloseTime.time_since_epoch().count();
+        return env.current()
+            ->header()
+            .parentCloseTime.time_since_epoch()
+            .count();
     }
 
     static auto