[SPARK-30725][SQL] Make legacy SQL configs as internal configs

### What changes were proposed in this pull request?
All legacy SQL configs are marked as internal configs. In particular, the following configs are updated as internals:
- spark.sql.legacy.sizeOfNull
- spark.sql.legacy.replaceDatabricksSparkAvro.enabled
- spark.sql.legacy.typeCoercion.datetimeToString.enabled
- spark.sql.legacy.looseUpcast
- spark.sql.legacy.arrayExistsFollowsThreeValuedLogic

### Why are the changes needed?
In general case, users shouldn't change legacy configs, so, they can be marked as internals.

### Does this PR introduce any user-facing change?
No

### How was this patch tested?
Should be tested by jenkins build and run tests.

Closes #27448 from MaxGekk/legacy-internal-sql-conf.

Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

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

@@ -1916,13 +1916,15 @@ object SQLConf {
     .createWithDefault(Deflater.DEFAULT_COMPRESSION)
 
   val LEGACY_SIZE_OF_NULL = buildConf("spark.sql.legacy.sizeOfNull")
+    .internal()
     .doc("If it is set to true, size of null returns -1. This behavior was inherited from Hive. " +
       "The size function returns null for null input if the flag is disabled.")
     .booleanConf
     .createWithDefault(false)
 
   val LEGACY_REPLACE_DATABRICKS_SPARK_AVRO_ENABLED =
     buildConf("spark.sql.legacy.replaceDatabricksSparkAvro.enabled")
+      .internal()
       .doc("If it is set to true, the data source provider com.databricks.spark.avro is mapped " +
         "to the built-in but external Avro data source module for backward compatibility.")
       .booleanConf
@@ -2048,10 +2050,11 @@ object SQLConf {
 
   val LEGACY_CAST_DATETIME_TO_STRING =
     buildConf("spark.sql.legacy.typeCoercion.datetimeToString.enabled")
+      .internal()
       .doc("If it is set to true, date/timestamp will cast to string in binary comparisons " +
         "with String")
-    .booleanConf
-    .createWithDefault(false)
+      .booleanConf
+      .createWithDefault(false)
 
   val DEFAULT_CATALOG = buildConf("spark.sql.defaultCatalog")
     .doc("Name of the default catalog. This will be the current catalog if users have not " +
@@ -2071,6 +2074,7 @@ object SQLConf {
       .createOptional
 
   val LEGACY_LOOSE_UPCAST = buildConf("spark.sql.legacy.looseUpcast")
+    .internal()
     .doc("When true, the upcast will be loose and allows string to atomic types.")
     .booleanConf
     .createWithDefault(false)
@@ -2083,6 +2087,7 @@ object SQLConf {
 
   val LEGACY_ARRAY_EXISTS_FOLLOWS_THREE_VALUED_LOGIC =
     buildConf("spark.sql.legacy.arrayExistsFollowsThreeValuedLogic")
+      .internal()
       .doc("When true, the ArrayExists will follow the three-valued boolean logic.")
       .booleanConf
       .createWithDefault(true)