### What changes were proposed in this pull request?
Change the definition of `findTightestCommonType` from
```
def findTightestCommonType(t1: DataType, t2: DataType): Option[DataType]
```
to
```
val findTightestCommonType: (DataType, DataType) => Option[DataType]
```
### Why are the changes needed?
For backward compatibility.
When running a MongoDB connector (built with Spark 3.1.1) with the latest master, there is such an error
```
java.lang.NoSuchMethodError: org.apache.spark.sql.catalyst.analysis.TypeCoercion$.findTightestCommonType()Lscala/Function2
```
from https://github.com/mongodb/mongo-spark/blob/master/src/main/scala/com/mongodb/spark/sql/MongoInferSchema.scala#L150
In the previous release, the function was
```
static public scala.Function2<org.apache.spark.sql.types.DataType, org.apache.spark.sql.types.DataType, scala.Option<org.apache.spark.sql.types.DataType>> findTightestCommonType ()
```
After https://github.com/apache/spark/pull/31349, the function becomes:
```
static public scala.Option<org.apache.spark.sql.types.DataType> findTightestCommonType (org.apache.spark.sql.types.DataType t1, org.apache.spark.sql.types.DataType t2)
```
This PR is to reduce the unnecessary API change.
### Does this PR introduce _any_ user-facing change?
Yes, the definition of `TypeCoercion.findTightestCommonType` is consistent with previous release again.
### How was this patch tested?
Existing unit tests
Closes#32493 from gengliangwang/typecoercion.
Authored-by: Gengliang Wang <ltnwgl@gmail.com>
Signed-off-by: Gengliang Wang <ltnwgl@gmail.com>
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