Update diff.groovy to work with latest GROOVY version

Currently we are using the older groovy version 2.4.8 we should update it with the latest

This is the command line output when we run script on groovy version 4.0.5
```
kevin@inspiron-15-5510:~/Desktop/check_style/checkstyle/contribution/checkstyle-tester
$ groovy diff.groovy --localGitRepo /home/kevin/Desktop/check_style/checkstyle/checkstyle \
--baseBranch master --patchBranch ParenPad2 \
--config /home/kevin/Desktop/check_style/checkstyle/confinff.xml \
--listOfProjects projects-to-test-on.properties

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
/home/kevin/Desktop/check_style/checkstyle/contribution/checkstyle-tester/diff.groovy: 53:
unable to resolve class CliBuilder
@ line 53, column 15.
def cli = new CliBuilder(usage:'groovy diff.groovy [options]', header: ....
^

/home/kevin/Desktop/check_style/checkstyle/contribution/checkstyle-tester/diff.groovy: 650:
unable to resolve class AntBuilder
@ line 650, column 5.
new AntBuilder().replace(
^

/home/kevin/Desktop/check_style/checkstyle/contribution/checkstyle-tester/diff.groovy: 658:
unable to resolve class AntBuilder
@ line 658, column 5.
new AntBuilder().copy(todir: destination) {
^

/home/kevin/Desktop/check_style/checkstyle/contribution/checkstyle-tester/diff.groovy: 664:
unable to resolve class AntBuilder
@ line 664, column 5.
new AntBuilder().move(todir: destination) {
^

/home/kevin/Desktop/check_style/checkstyle/contribution/checkstyle-tester/diff.groovy: 670:
unable to resolve class AntBuilder
@ line 670, column 5.
new AntBuilder().delete(dir: dir, failonerror: false)
^

5 errors
```