fix: ClassCastException in Spring Data JDBC sample (#1595)

Author: olavloite

samples/spring-data-jdbc/src/main/java/com/google/cloud/spanner/sample/JdbcConfiguration.java

@@ -64,7 +64,7 @@ public static boolean isCloudSpannerPG(JdbcOperations operations) {
           "The selected Cloud Spanner database does not use the PostgreSQL dialect");
     } catch (DataAccessException exception) {
       if (exception.getCause() instanceof JdbcSqlException) {
-        JdbcSqlException jdbcSqlException = (JdbcSqlException) exception;
+        JdbcSqlException jdbcSqlException = (JdbcSqlException) exception.getCause();
         if (jdbcSqlException.getCode() == Code.PERMISSION_DENIED
             || jdbcSqlException.getCode() == Code.NOT_FOUND) {
           throw new RuntimeException(