Rename and extract Context. (#8528)

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

tests/cpp/gbm/test_gblinear.cc

@@ -7,11 +7,11 @@
 #include <sstream>
 
 #include "../helpers.h"
-#include "xgboost/json.h"
-#include "xgboost/logging.h"
+#include "xgboost/context.h"
 #include "xgboost/gbm.h"
-#include "xgboost/generic_parameters.h"
+#include "xgboost/json.h"
 #include "xgboost/learner.h"
+#include "xgboost/logging.h"
 
 namespace xgboost {
 namespace gbm {

tests/cpp/gbm/test_gbtree.cc

@@ -2,7 +2,7 @@
  * Copyright 2019-2022 XGBoost contributors
  */
 #include <gtest/gtest.h>
-#include <xgboost/generic_parameters.h>
+#include <xgboost/context.h>
 
 #include "../../../src/data/adapter.h"
 #include "../../../src/data/proxy_dmatrix.h"