[CORE] Remove `.sql.columnar` from configuration keys for simplicity

Part of https://github.com/apache/incubator-gluten/issues/6970.

For example: `spark.gluten.sql.columnar.tableCache` can be simplified to `spark.gluten.tableCache` by removing `.sql.columnar`.

We'll still keep the the old keys available for users for backward-compatibility. But while the old keys are specified, raise a warning showing it's deprecated.