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/rpc/LedgerRequest_test.cpp

@@ -32,7 +32,7 @@ public:
 
         env.close();
         env.close();
-        BEAST_EXPECT(env.current()->info().seq == 5);
+        BEAST_EXPECT(env.current()->header().seq == 5);
 
         {
             // arbitrary text is converted to 0.