Deprecate AssertFile

[fmbenhassine]

org.springframework.batch.test.AssertFile provides two static methods to compare two files and assert on a file line count. Those two utilities can be found in modern test tools like JUnit 5 (Assertions.assertLinesMatch()) or AssertJ (Assertions.assertThat(actualFile).hasSameTextualContentAs(expectedFile);).

The goal of spring-batch-test module is to provide utilities that are specific to Spring Batch and not general assertion utilities on files or databases. AssertFile should be deprecated in 5.0 and removed in 5.2 in favor of modern test libraries.