Simplify Spark version specific Scala code (#333)

Some classes do not have alternatives for other Spark versions, so they are moved into `src/main/scala`.

Removes `UnixMicros.unixMicros`, use `org.apache.spark.sql.catalyst.expressions.UnixMicros` instead.

Author: EnricoMi

src/main/scala-spark-3.2/uk/co/gresearch/spark/BinaryLikeWithNewChildrenInternal.scala

@@ -20,7 +20,7 @@ import org.apache.spark.sql.catalyst.expressions.Expression
 
 package object extension {
   implicit class ColumnExtension(col: Column) {
-    // Column.expr exists in this Spark version
+    // Column.expr exists in this Spark version and earlier
     def sql: String = col.expr.sql
   }