UUIDSerializer: Change type cast to parent type to fix issues using w…

…ithin a list.

The UUIDSerializer's deserializer will fail with a `ClassCastException` when the UUID is embedded in a list (e.g., `val foo: Collection<@contextual UUID> = emptyList()`).  The decoder in this example is a `ListDecoder` rather than the expected `BsonFlexibleDecoder` triggering the exception.  Both, however, are `FlexibleDecoder` objects and the reader field is declared there.  Updated the cast to this parent type, resolving the issue.

kmongo-serialization-mapping/src/main/kotlin/Serializers.kt

@@ -346,7 +346,7 @@ internal object UUIDSerializer : KSerializer<UUID> {
     override val descriptor: SerialDescriptor = buildClassSerialDescriptor("UuidSerializer")
 
     override fun deserialize(decoder: Decoder): UUID =
-        (decoder as BsonFlexibleDecoder).reader.readBinaryData().asUuid()
+        (decoder as FlexibleDecoder).reader.readBinaryData().asUuid()
 
     override fun serialize(encoder: Encoder, value: UUID) {
         (encoder as BsonEncoder).encodeUUID(value)