chore: use serializable as default isolation level (#2040)

Author: olavloite

samples/spring-data-jdbc/googlesql/src/main/resources/application.properties

@@ -7,7 +7,7 @@ spanner.database=spring-data-jdbc
 
 # Sets the isolation level that will be used by default for read/write transactions.
 # Spanner supports the isolation levels SERIALIZABLE and REPEATABLE READ.
-spanner.default_isolation_level=REPEATABLE_READ
+spanner.default_isolation_level=SERIALIZABLE
 
 # The sample by default uses the Spanner emulator.
 # Disable this flag to run the sample on a real Spanner instance.

samples/spring-data-mybatis/googlesql/src/main/resources/application.properties

@@ -21,7 +21,7 @@ spanner.database=mybatis-sample
 
 # Sets the isolation level that will be used by default for read/write transactions.
 # Spanner supports the isolation levels SERIALIZABLE and REPEATABLE READ.
-spanner.default_isolation_level=REPEATABLE_READ
+spanner.default_isolation_level=SERIALIZABLE
 
 spring.datasource.url=jdbc:cloudspanner:${spanner.endpoint}/projects/${spanner.project}/instances/${spanner.instance}/databases/${spanner.database};default_isolation_level=${spanner.default_isolation_level};autoConfigEmulator=${spanner.emulator};${spanner.additional_properties}
 spring.datasource.driver-class-name=com.google.cloud.spanner.jdbc.JdbcDriver