Feature/integration tests with testcontainers

[sdbuehlmann]

• Deleted "old" integration tests (which needed external databases to be started).
• Added new integration tests that use TestContainers.
• Removed GitHub Actions that used Ant.

[sdbuehlmann]

Dear @mburri, thanks for your valuable feedback. I've implemented all of your inputs except one: I've never deleted the doc-directory. I've deleted the target-directory, which, as far as I know, is a build-output-directory containing just temporary files. There were just duplicated files in it, so I thought that it should be okay to delete it.

Could you please review the PR again?

Please note the following things:

• I've removed all Ant-specific files.
• Creating "the mysterious" ZIP folder, which contains all scripts, jars, and documentation, can be done with "./gradlew packDeliverables."
• I've split the Build-Action into a Continuous-Build-Action (just executing the unit tests) and an Integration-Build-Action (which executes the slow integration tests).
• I've copied the release workflow (including the GitHub action and the used Gradle Plugin) from the SIARD-Suite. Please take a look at my changes (compared to the SIARD-Suite) in the deliverables.yml file.
• The MANIFEST will no longer be copied but created by Gradle. This caused some failed tests. Due to that, I've changed the way the application will read its own version. Please take a close look at the VersionsExplorer.class and the createVersionsPropertiesFile Gradle task. I am looking forward to your feedback on that.