GH-242: Delete original zip only if no errors

Fixes https://github.com/spring-projects/spring-integration-extensions/issues/242

The `UnZipTransformer` removes a source zip file unconditionally

* Fix `UnZipTransformer` to remove a source zip file only
if unzipping was successful
* More checks for traversal file names
* Upgrade dependencies including Gradle
* Move Maven publishing functionality to `maven-publish` plugin

spring-integration-zip/gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.10.2-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.8.3-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists