Rename and extract Context. (#8528)

* Rename `GenericParameter` to `Context`.
* Rename header file to reflect the change.
* Rename all references.

tests/cpp/objective/test_hinge.cc

@@ -1,14 +1,14 @@
 // Copyright by Contributors
 #include <xgboost/objective.h>
-#include <xgboost/generic_parameters.h>
+#include <xgboost/context.h>
 #include <limits>
 
 #include "../helpers.h"
 
 TEST(Objective, DeclareUnifiedTest(HingeObj)) {
-  xgboost::GenericParameter tparam = xgboost::CreateEmptyGenericParam(GPUIDX);
+  xgboost::Context ctx = xgboost::CreateEmptyGenericParam(GPUIDX);
   std::unique_ptr<xgboost::ObjFunction> obj {
-    xgboost::ObjFunction::Create("binary:hinge", &tparam)
+    xgboost::ObjFunction::Create("binary:hinge", &ctx)
   };
 
   xgboost::bst_float eps = std::numeric_limits<xgboost::bst_float>::min();