[SPARK-29363][MLLIB] Make o.a.s.regression.Regressor public

### What changes were proposed in this pull request?

- Removal of `private[ml]` modifier from `Regressor`.
- Marking `Regressor` as  `DeveloperApi`.

### Why are the changes needed?
Consistency with the rest of ML API as described in [the corresponding JIRA ticket](https://issues.apache.org/jira/browse/SPARK-29363).

### Does this PR introduce any user-facing change?
Yes, as described above.

### How was this patch tested?
Existing tests.

Closes #26033 from zero323/SPARK-29363.

Authored-by: zero323 <mszymkiewicz@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

mllib/src/main/scala/org/apache/spark/ml/regression/Regressor.scala

@@ -28,7 +28,8 @@ import org.apache.spark.ml.{PredictionModel, Predictor, PredictorParams}
  * @tparam Learner  Concrete Estimator type
  * @tparam M  Concrete Model type
  */
-private[spark] abstract class Regressor[
+@DeveloperApi
+abstract class Regressor[
     FeaturesType,
     Learner <: Regressor[FeaturesType, Learner, M],
     M <: RegressionModel[FeaturesType, M]]