[R] Add a compatibility layer to load Booster object from an old RDS file (#5940)

* [R] Add a compatibility layer to load Booster from an old RDS
* Modify QuantileHistMaker::LoadConfig() to be backward compatible with 1.1.x
* Add a big warning about compatibility in QuantileHistMaker::LoadConfig()
* Add testing suite
* Discourage use of saveRDS() in CRAN doc

R-package/tests/generate_models_params.R

@@ -0,0 +1,10 @@
+model_generator_metadata <- function() {
+  return (list(
+    kRounds = 2,
+    kRows = 1000,
+    kCols = 4,
+    kForests = 2,
+    kMaxDepth = 2,
+    kClasses = 3
+  ))
+}