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/Vault_test.cpp

@@ -3520,7 +3520,7 @@ class Vault_test : public beast::unit_test::suite
         {
             testcase("private vault expired authorization");
             uint32_t const closeTime = env.current()
-                                           ->info()
+                                           ->header()
                                            .parentCloseTime.time_since_epoch()
                                            .count();
             {