Use type safe JdbcClient query

Update `JdbcClientAutoConfigurationTests` to use a type safe query
since Spring Framework now returns `Object` types when no type is
specified (see Spring Framework issue 31403).

See gh-37710

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/jdbc/JdbcClientAutoConfigurationTests.java

@@ -94,7 +94,7 @@ class JdbcClientAutoConfigurationTests {
 		private final Long count;
 
 		JdbcClientDataSourceMigrationValidator(JdbcClient jdbcClient) {
-			this.count = jdbcClient.sql("SELECT COUNT(*) from CITY").query().singleValue();
+			this.count = jdbcClient.sql("SELECT COUNT(*) from CITY").query(Long.class).single();
 		}
 
 	}