[SPARK-44718][SQL] Match ColumnVector memory-mode config default to O…

…ffHeapMemoryMode config value

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

Set the column vector default memory mode to depend on the off-heap memory mode flag. This is to prevent a user from using Vectorized-Reader with an on-heap column-vector by default when the off-heap memory mode is enabled on the cluster.

### Why are the changes needed?
Avoid the unintentional usage of on-heap memory in vectorized-reader when off-heap memory mode is enabled by the user.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Manual & existing tests.

Closes apache#42394 from majdyz/offheap-colvec-mode-default-value.

Lead-authored-by: Zamil Majdy <[email protected]>
Co-authored-by: Zamil Majdy <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>
(cherry picked from commit afcccb4)

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -477,10 +477,10 @@ object SQLConf {
   val COLUMN_VECTOR_OFFHEAP_ENABLED =
     buildConf("spark.sql.columnVector.offheap.enabled")
       .internal()
-      .doc("When true, use OffHeapColumnVector in ColumnarBatch.")
+      .doc("When true, use OffHeapColumnVector in ColumnarBatch. " +
+        s"Defaults to $MEMORY_OFFHEAP_ENABLED.")
       .version("2.3.0")
-      .booleanConf
-      .createWithDefault(false)
+      .fallbackConf(MEMORY_OFFHEAP_ENABLED)
 
   val PREFER_SORTMERGEJOIN = buildConf("spark.sql.join.preferSortMergeJoin")
     .internal()