Improve Kotlin Testcontainers samples

Make those code samples more idiomatic.

See gh-35446

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/features/testing/testcontainers/dynamicproperties/MyIntegrationTests.kt

@@ -26,7 +26,7 @@ import org.testcontainers.junit.jupiter.Testcontainers
 
 @SpringBootTest
 @Testcontainers
-internal class MyIntegrationTests {
+class MyIntegrationTests {
 
 	@Test
 	fun myTest() {
@@ -36,7 +36,7 @@ internal class MyIntegrationTests {
 	companion object {
 
 		@Container
-		var neo4j: Neo4jContainer<*> = Neo4jContainer<Nothing>("neo4j:5")
+		val neo4j = Neo4jContainer("neo4j:5")
 
 		@DynamicPropertySource
 		fun neo4jProperties(registry: DynamicPropertyRegistry) {

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/features/testing/testcontainers/serviceconnections/MyIntegrationTests.kt

@@ -26,7 +26,7 @@ import org.springframework.boot.testcontainers.service.connection.ServiceConnect
 
 @SpringBootTest
 @Testcontainers
-internal class MyIntegrationTests {
+class MyIntegrationTests {
 
 	@Test
 	fun myTest() {
@@ -37,7 +37,7 @@ internal class MyIntegrationTests {
 
 		@Container
 		@ServiceConnection
-		var neo4j: Neo4jContainer<*> = Neo4jContainer<Nothing>("neo4j:5")
+		val neo4j = Neo4jContainer("neo4j:5")
 
 	}
 

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/features/testing/testcontainers/vanilla/MyIntegrationTests.kt

@@ -24,7 +24,7 @@ import org.testcontainers.junit.jupiter.Testcontainers
 
 @SpringBootTest
 @Testcontainers
-internal class MyIntegrationTests {
+class MyIntegrationTests {
 
 	@Test
 	fun myTest() {
@@ -33,7 +33,7 @@ internal class MyIntegrationTests {
 
 	companion object {
 		@Container
-		var neo4j: Neo4jContainer<*> = Neo4jContainer<Nothing>("neo4j:5")
+		val neo4j = Neo4jContainer("neo4j:5")
 	}
 
 }