Deprecate gpu_exact, bump required cuda version in docs (#4527)

src/tree/updater_gpu.cu

@@ -830,7 +830,11 @@ class GPUMaker : public TreeUpdater {
 
 XGBOOST_REGISTER_TREE_UPDATER(GPUMaker, "grow_gpu")
     .describe("Grow tree with GPU.")
-    .set_body([]() { return new GPUMaker(); });
+    .set_body([]() {
+      LOG(WARNING) << "The gpu_exact tree method is deprecated and may be "
+                      "removed in a future version.";
+      return new GPUMaker();
+    });
 
 }  // namespace tree
 }  // namespace xgboost