Upgrade to Kotlin 1.4 M2

- The compiler is configured to retain compatibility with Kotlin 1.3.
- Explicit API mode is not yet enabled but could be in the future.
- A workaround for Gradle build is required for now, see
  https://youtrack.jetbrains.com/issue/KT-39610 for more details.
- Some exceptions thrown by Kotlin have changed to NullPointerException,
  see https://youtrack.jetbrains.com/issue/KT-22275 for more details.

Closes gh-24171

spring-jdbc/src/test/kotlin/org/springframework/jdbc/core/JdbcOperationsExtensionsTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors
+ * Copyright 2002-2020 the original author or authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -53,7 +53,7 @@ class JdbcOperationsExtensionsTests {
 	@Test  // gh-22682
 	fun `queryForObject with nullable RowMapper-like function`() {
 		every { template.queryForObject(sql, any<RowMapper<Int>>(), 3) } returns null
-		assertThat(template.queryForObject(sql, 3) { _, _ -> null as Int? }).isNull()
+		assertThat(template.queryForObject<Int?>(sql, 3) { _, _ -> null }).isNull()
 		verify { template.queryForObject(eq(sql), any<RowMapper<Int?>>(), eq(3)) }
 	}
 
@@ -130,7 +130,7 @@ class JdbcOperationsExtensionsTests {
 
 	@Test
 	fun `query with RowMapper-like function`() {
-		val list = listOf(1, 2, 3)
+		val list = mutableListOf(1, 2, 3)
 		every { template.query(sql, ofType<RowMapper<*>>(), 3) } returns list
 		assertThat(template.query(sql, 3) { rs, _ ->
 			rs.getInt(1)